Insect bite's upset (4)
I believe the answer is:
gnat
'insect' is the definition.
(gnat is a kind of insect)
'bite's upset' is the wordplay.
'bite' becomes 'tang' (I've seen this before).
'upset' is a reversal indicator.
'tang' backwards is 'GNAT'.
(Other definitions for gnat that I've seen before include "Small biting fly" , "Insect often hovering over stagnant water" , "Small fly that bites" , "insect's" , "Tang (anag.)" .)