Regret very much pronounced in French philosopher (8)

I believe the answer is:

rousseau

'french philosopher' is the definition.
(18th-century philosopher Jean-Jacques Rousseau)

I can't explain the remainder of the clue.

(Other definitions for rousseau that I've seen before include "Arouse us like Jean-Jacques . . . . . . . ., philosopher" , "He did paintings" , "Jean-Jacques --, philosopher and writer" , "Jean-Jacques --, Swiss-French philosopher" , "French philosopher or painter" .)

I've seen this clue in The Times.

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