Tiny biter (4)
I believe the answer is:
flea
'tiny biter' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for flea that I've seen before include "Bloodsucking parasite" , "Small blood-sucking insect" , "Jumping parasite" , "Small parasitic insect" , "Small jumper in small circus" .)