Testes injured in fly (6)
I believe the answer is:
tsetse
'fly' is the definition.
(tsetse is a kind of fly)
'testes injured' is the wordplay.
'injured' indicates anagramming the letters.
'testes' with letters rearranged gives 'TSETSE'.
'in' acts as a link.
(Other definitions for tsetse that I've seen before include "carrier of disease" , "flies" , "source of sleeping sickness" , "Bloodsucking African fly" , "Disease-carrying fly" .)