Insect bite in reverse (4)
I believe the answer is:
gnat
'insect' is the definition.
(gnat is a kind of insect)
'bite in reverse' is the wordplay.
'bite' becomes 'tang' (I've seen this before).
'in reverse' says the letters should be written backwards.
'tang' in reverse letter order is 'GNAT'.
(Other definitions for gnat that I've seen before include "Two-winged fly" , "Something with bite" , "Tang of a biting fly" , "Something that bites" , "Any of various small biting flies" .)