Small insect, beginning to nip, bit (7)
I believe the answer is:
smidgen
'bit' is the definition.
(I know that bit can be written as smidgen)
'small insect beginning to nip' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'insect' becomes 'midge' (midge is a kind of insect).
'beginning to' suggests taking the first letters.
The first letter of 'nip' is 'n'.
's'+'midge'+'n'='SMIDGEN'
(Other definitions for smidgen that I've seen before include "Tiny part" , "Scarcely detectable amount" , "Very little" , "Taste" , "Iota" .)