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