The hosts responded with a series of powerful drives in which Sebastien Chabal and Lionel Nallet were prominent and which prompted Martyn Williams to kill the ball illegally for Parra to level the scores after six minutes.
Clerc skated over in between a brace from giant Castres second row Nallet, and four Elissalde conversions meant the hosts led 40-3 at half-time with the bonus point already safely tucked away.