Parasitic insects (4)
I believe the answer is:
lice
'parasitic insects' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for lice that I've seen before include "Parasitic insects" , "Wingless bloodsucking parasites" , "Crawlers" , "Bloodsucking insects" , "Bloodsucking parasitic insect" .)