Small insect bit (6)
I believe the answer is:
smidge
'bit' is the definition.
(I've seen this in another clue)
'small insect' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'insect' becomes 'midge' (midge is a kind of insect).
's'+'midge'='SMIDGE'
(Other definitions for smidge that I've seen before include "very small part" , "Really tiny amount" .)