Insect bite upsetting (4)
I believe the answer is:
gnat
'insect' is the definition.
(I know that gnat is a type of dipteran)
'bite upsetting' is the wordplay.
'bite' becomes 'tang' (both can mean a flavour or taste).
'upsetting' is a reversal indicator.
'tang' reversed gives 'GNAT'.
(Other definitions for gnat that I've seen before include "Small (swarming) fly" , "Stinging insect" , "Tang of a biting fly" , "Small biting flying insect" , "Two-winged fly" .)