Parasite found in genitals (3)
I believe the answer is:
nit
'parasite' is the definition.
(thesaurus)
'in genitals' is the wordplay.
'in' indicates the answer is hidden within the clue.
'NIT' is hidden in the letters of 'genitals'.
'found' is the link.
(Other definitions for nit that I've seen before include "Egg of a louse (3)" , "Egg; fool" , "Louse; fool" , "Louse-egg" , "Foolish person" .)