The French comedian entertains us, mounting action (7)
I believe the answer is:
lawsuit
'action' is the definition.
('lawsuit' can be a synonym of 'action')
'the french comedian entertains us mounting' is the wordplay.
'the french' becomes 'la'.
'comedian' becomes 'wit' (I've seen this before).
'entertains' indicates putting letters inside.
'mounting' says the letters should be written in reverse (going up in a down clue).
'us' in reverse letter order is 'su'.
'wit' going around 'su' is 'wsuit'.
'la'+'wsuit'='LAWSUIT'
(Other definitions for lawsuit that I've seen before include "The process of bringing action in legal court" , "Process of bringing dispute or claim before the courts" , "Court case" , "Legal dispute" , "Claim brought to court" .)