Castres stay in the hunt
Castres stay in the huntSHARE
The visitors' seventh consecutive victory came through tries from Florian Vialelle, Samoan international David Smith and Alex Tulou before Ludovic Radosavljevic applied the killer blow to his former club with a 72nd-minute effort that put Castres ahead for the first time.
Despite intense Clermont pressure during eight minutes of injury time, Castres emerged with a victory that leaves them third, level on points with second-placed Montpellier and two behind leaders La Rochelle, who crushed struggling Agen 47-6 on Saturday.
"This victory rounds off a great 2017 for us," said Castres coach Christophe Urios.
Clermont, already depleted by injuries, suffered more frustration when former England wing David Strettle hurt his ankle early in the game, and Aurelien Rougerie suffered a knee injury.
"I can't be angry with my players [for the defeat], because we are really feeling the impact physically at the moment," Clermont coach Franck Azema said.
"We are still going to try to qualify for the Top 6 but we need all our weapons to do that and today we may have lost two or even three players."
Lyon beat Pau in the day's other match, getting back on track after a damaging run of five defeats in November and December.
Lionel Beauxis was Lyon's driving force, scoring a try and kicking efficiently to help land a 35-23 win that puts his side seventh on 37 points going into 2018.
On Saturday, La Rochelle bounced back from successive defeats as they retook first place by showing no mercy to Agen.
La Rochelle, who had lost at Bordeaux-Begles and against Wasps in the European Champions Cup, scored seven tries and picked up a bonus point.
The big win came against an Agen team showing 15 changes from the side that had won against Brive last weekend.
Agen kicked both their penalties in the first 13 minutes, but by then Jeremy Sinzelle had scored the home team's first try.
La Rochelle pounded away and forced holes in a disorganised Agen defence. Hooker Pierre Bourgarit went over after 24 minutes and Alexi Bales found a gap and raced 40 metres for another try before half-time.
Gregory Lamboley, Aguillon, Afa Amosa and Pierre Boudehent added further touchdowns in the second half.
Montpellier made changes to their team and paid a heavy price, as Brive ran in four tries, including two from Nadir Megdoud, to claim a bonus-point victory that lifts them above Agen to third-bottom spot.
The visitors went 10-7 ahead early in the second half when Benjamin Fall scored a try converted by Benoit Paillaugue.
However, they then conceded 22 points without reply to slump to defeat, to the frustration of boss Vern Cotter.
"It is a good lesson for us to keep working to the end, maintain a game plan and put pressure on the opposition, especially against a team like Brive who needed points," Cotter said.
Racing 92 are up to third thanks to a narrow 16-12 victory away at bottom side Oyonnax, with Marc Andreu crossing for the only try of the game and Teddy Iribaren kicked the rest of the points for Racing.
Meanwhile, Bordeaux-Begles bid adieu to coach Jacques Brunel, who is taking over the French national team, with a 22-12 loss at Stade Francais in Paris where Waisea Nayacalevu secured a bonus point for the home team with a 77th-minute try.
Brunel will officially start his new job as the national team boss next week after replacing the sacked Guy Noves last Wednesday, just five weeks ahead of France's Six Nations opener against Ireland at the Stade de France.
Toulouse beat Toulon 18-13 and ended the weekend in sixth position, seven points behind La Rochelle.