Dances quietly, held by leader mostly (4)
I believe the answer is:
bops
'dances' is the definition.
(bopping is a kind of dancing)
'quietly held by leader mostly' is the wordplay.
'quietly' becomes 'p' (p for piano and quiet).
'held by' indicates putting letters inside.
'leader' becomes 'boss' (boss is a kind of leader).
'mostly' means to remove the last letter (most but not all of the word).
'boss' with its final letter removed is 'bos'.
'p' inserted into 'bos' is 'BOPS'.
(Other definitions for bops that I've seen before include "Dances to pop" , "hits" .)