'Act proper' for man celebrating to some extent (11)
I believe the answer is:
performance
'act' is the definition.
(performance is a kind of act)
'proper for man celebrating to some extent' is the wordplay.
'to some extent' says the answer is hidden in the clue.
'PERFORMANCE' is hidden in the letters of 'proper for man celebrating'.
(Other definitions for performance that I've seen before include "(Actor's) rendering of a role" , "Particular action" , "The giving of a play or concert" , "Stage show" , "Dramatic production" .)