Parasite in rotten leaf (4)
I believe the answer is:
flea
'parasite' is the definition.
(I know that flea is a type of ectoparasite)
'rotten leaf' is the wordplay.
'rotten' indicates anagramming the letters.
'leaf' is an anagram of 'FLEA'.
'in' is the link.
(Other definitions for flea that I've seen before include "Jumping bloodsucker" , "Small jumper in small circus" , "In your ear, this insect is a rebuke" , "Insect (which you don't want in your ear!)" , "In one's ear, it's a stinging rebuke" .)