Returning, say - stupid for an insect (5)
I believe the answer is:
midge
'for an insect' is the definition.
(midge is a kind of insect)
'returning say stupid' is the wordplay.
'returning' shows that the letters should be reversed in order.
'say' becomes 'eg' (short for 'exempli gratia', 'for example' in Latin).
'stupid' becomes 'dim' (similar in meaning).
'eg'+'dim'='egdim'
'egdim' in reverse letter order is 'MIDGE'.
(Other definitions for midge that I've seen before include "Small, gnat-like fly" , "Tiny biting fly" , "A small biting gnat-like insect" , "Small flying insect" , "Tiny swarming fly" .)