A little lad traps fly (8)
I believe the answer is:
smidgeon
'a little' is the definition.
(I've seen this before)
'lad traps fly' is the wordplay.
'lad' becomes 'son' (lad can informally mean a young boy).
'traps' indicates putting letters inside.
'fly' becomes 'midge' (I've seen this before).
'son' enclosing 'midge' is 'SMIDGEON'.
(Other definitions for smidgeon that I've seen before include "Small portion" , "bit" , "Small amount" , "Tiny amount" , "Very small part" .)