Castres plunge Toulon deeper into crisis
Four-try Clermont were in control 29-10 at the break only for their rock-bottom hosts to hit back on their artificial pitch to seal a 32-32 draw to end a run of seven consecutive defeats.
Clermont scrumhalf Morgan Parra, who scored his side's only second-half points, a penalty, reflected: "When you have such a strong lead you can't be left trying to win the game by just one point at the last minute."
Elsewhere La Rochelle beat Pau 44-14 to move second, Stade Francais were downed 13-29 at Agen, while Bordeaux-Begles and Brive drew 27-27.
Toulon scored three tries to two from the home team, but only kicked four points, while Benjamin Urdapilleta struck two penalties and two conversions for Castres.
Julien Caminati gave the home team the lead with a third-minute try before Chris Ashton and Ma'a Nonu quickly replied. Toulon led by two at half-time.
They led 19-13 after a try by Malakai Fekitoa in the 57th minute but Emerick Setiano, a prop who had just come off the bench, received a yellow card in the 59th minute.
Six minutes later, Castres won a penalty try after a Toulon collapsed a maul. Urdapilleta converted to put the home team one point ahead.
It was a fourth straight victory for Castres who climbed to fourth in the table and a third consecutive league defeat for Toulon who are eighth.
"I do not think the confidence of the squad has been dented. Its a united group," said Fabrice Landreau, the Toulon coach. "Even so, we must still start winning points."
On Sunday, leaders Montpellier face a test of their title credentials against 2016 champions Racing 92.
And Lyon can reclaim second from La Rochelle if things go their way at home to Toulouse.