Clermont Auvergne made it three wins from three in this season's Heineken Cup with a 15-12 victory over defending champions Leinster at the Stade Marcel Michelin.

Despite seeing the end of their 17-game unbeaten run in the competition, Leinster emerged with what could be a crucial bonus point and a clutch of positives ahead of next Saturday's return date in Dublin. Sean O'Brien had an impressive return to European action in a barnstorming back row unit, with Fergus McFadden and cup debutant Andrew Goodman the pick of the visitors' backs.

Jonathan Sexton kicked four penalties for the province, a tally matched by Clermont's Morgan Parra, with Brock James landing a crucial drop goal just before half-time.

There was a raucous atmosphere in Clermont's home fortress, where the French club boast an unbeaten run that recently passed 50 games. Clermont drew first blood, Parra popping over a penalty from in front of the posts but Leinster carried more of an early threat with ball in hand.

Jamie Heaslip's inside pass put Ian Madigan through a midfield hole and Sexton stepped up to convert the subsequent penalty. An obstruction from a lineout went unpunished as Sexton missed a tricky penalty, with play swinging back and forth at a hectic pace.

A strong burst from Napolioni Nalaga led to Parra kicking Clermont back in front by the end of a defence-dominated first quarter. The home side cranked it up as Sitiveni Sivivatu drew in two defenders, releasing Aurelien Rougerie and Lee Byrne for a right-wing surge that almost led to a try.

A third successful penalty from Parra stretched the margin to 9-3, but Leinster were quick to respond through a bout of sturdy carries from McFadden, Cian Healy and Kevin McLaughlin. Sexton was back on target with a penalty from the 22 and as the Leinster pack exerted more control, they continued to make good ground.

Madigan burst into the 22 and with Clermont penalised for not rolling away, Sexton brought the title holders level. However, Clermont hit back through another penalty from Parra and James' late drop goal put six points between the sides at the break.

Sexton and Madigan probed with clever kicks on the resumption, while Isa Nacewa and McFadden both went close to collecting cross-field kicks from the Leinster number 10. Wales and Lions fullback Byrne did brilliantly to deny the former.

After two confidence-boosting scrums, with Mike Ross shunting Vincent Debaty backwards, Sexton slotted his fourth successful penalty with 54 minutes on the clock. Clermont looked to be tiring but Byrne did well again to sweep the ball into touch with Nacewa bearing down on him as he chased Gordon D'Arcy's kick.

Although the French outfit responded with two side-stepping runs from the dangerous Wesley Fofana, Leinster defended stoutly but a couple of loose lineouts - with replacement hooker Richard Strauss on the pitch - robbed them of promising attacking positions as time ran down.

The result leaves Clermont with a five-point buffer at the top of the table and Leinster boss Joe Schmidt was frustrated his side did not get more out of the game. "I just think you come to the (Stade) Marcel Michelin and I think you've got to get over the line. You can't just go between the posts and hope to get something here," he said.

"I thought there were a couple of calls that maybe could have gone our way and we could have got something else between the posts. But the area that Johnny actually kicked out of when he cross-kicked was very, very heavy and muddy. It's not a guaranteed three. We did miss one and they didn't miss any. So that's how close these games are and it is really unfortunate."

The stage is now set for a pivotal second meeting at the Aviva Stadium next Saturday afternoon with Clermont coach Vern Cotter delighted to have bagged a vital win that extended his side's unbeaten home record to 51 games.

"We weren't particularly good in the second half and they were better in the second half than they were in the first half. We just did enough to stay ahead," he said. "The game was contested from start to finish very intensely, especially at the breakdown. They threw everything at us and I'm pleased that we managed to sneak away with the victory. Even if it was a skinny one, it was still four points and I think that will count in the end."