Quiet party, for sport (8)
I believe the answer is:
softball
'for sport' is the definition.
(softball is a kind of sport)
'quiet party' is the wordplay.
'quiet' becomes 'soft' (similar in meaning).
'party' becomes 'ball' (ball is a kind of party).
'soft'+'ball'='SOFTBALL'
(Other definitions for softball that I've seen before include "played by young American" , "game" , "Junior variant of baseball" , "sport!"" .)