Insect's smell picked up (4)
I believe the answer is:
gnat
'insect's' is the definition.
(gnat is a kind of insect)
'smell picked up' is the wordplay.
'smell' becomes 'tang' (I have seen 'Strong smell' mean 'tang' so perhaps 'smell' could also mean 'tang').
'picked up' shows that the letters should be reversed in order.
'tang' back-to-front is 'GNAT'.
(Other definitions for gnat that I've seen before include "Something that bites" , "Small, swarming insect" , "small irritant" , "mozzie" , "Tiny biting insect" .)