Jumping parasite (4)
I believe the answer is:
flea
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">jumping parasite</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)