Small fly found in child's trifle (8)
I believe the answer is:
smidgeon
'trifle' is the definition.
(both can mean a small amount)
'small fly found in child's' is the wordplay.
'small fly' becomes 'midge' (I've seen this before).
'found in' is an insertion indicator.
'child' becomes 'son' (son is a kind of child).
'midge' inserted inside 'son' is 'SMIDGEON'.
(Other definitions for smidgeon that I've seen before include "Very small part" , "Tiny amount" , "Small amount" , "Small portion" , "bit" .)