Son biting insect bit (6)
I believe the answer is:
smidge
'bit' is the definition.
(I know that bit can be written as smidge)
'son biting insect' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'biting insect' becomes 'midge' (I've seen this before).
's'+'midge'='SMIDGE'
(Other definitions for smidge that I've seen before include "Really tiny amount" , "very small part" .)