The French are in because it's quiet! (7)
I believe the answer is:
silence
'quiet' is the definition.
('silence' can be a synonym of 'quiet')
'the french are in because' is the wordplay.
'the french' becomes 'le'.
'are in' is an insertion indicator.
'because' becomes 'since' (I've seen this before).
'le' placed into 'since' is 'SILENCE'.
'it's' acts as a link.
(Other definitions for silence that I've seen before include "Soundlessness" , "Quiet" , "Absence of racket" , "may be required in court" , "Quietness" .)