Beagle biter (4)
I believe the answer is:
flea
'beagle 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 "Small blood-sucking insect" , "Wingless, jumping insect" , "Jumping, biting insect" , "Tiny parasite" , "Little jumper?" .)