The French must be in because there's quiet (7)
I believe the answer is:
silence
'quiet' is the definition.
(synonyms)
'the french must be in because' is the wordplay.
'the french' becomes 'le'.
'must be in' indicates putting letters inside.
'because' becomes 'since' (I've seen this before).
'le' put within 'since' is 'SILENCE'.
'there's' acts as a link.
(Other definitions for silence that I've seen before include "may be required in court" , "Lack of sound" , "Nothing said" , "Noiselessness" , "Hush!" .)