Fly back in great anguish (4)
I believe the answer is:
gnat
'fly' is the definition.
(type of insect)
'back in great anguish' is the wordplay.
'back' says the letters should be written in reverse.
'in' indicates a hidden word.
'tang' is hidden in the letters of 'great anguish'.
'tang' reversed gives 'GNAT'.
(Other definitions for gnat that I've seen before include "Small biting flying insect" , "Sucker (if female)" , "Insect often hovering over stagnant water" , "Any of various small biting flies" , "Biting pest" .)