Mischievous child’s quiet after racket (5)
I believe the answer is:
scamp
'mischievous child's' is the definition.
(I've seen this before)
'quiet after racket' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'after' is a charade indicator (letters next to each other).
'racket' becomes 'scam' (I've seen this before).
'p' after 'scam' is 'SCAMP'.
(Other definitions for scamp that I've seen before include "Mischievous child (colloq.)" , "pickle" , "Rogue - perform inadequately" , "villain" , "Little Devil" .)