Jumping parasite (4)
I believe the answer is:
flea
'jumping parasite' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for flea that I've seen before include "source of infestation" , "tiny jumper" , "irritant type" , "Leaping insects" , "In one's ear, it's a stinging rebuke" .)