Dog-biting bug (4)
I believe the answer is:
flea
'dog-biting bug' 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 "tiny jumper" , "one is proverbially fit" , "Jumping, blood-sucking insect" , "little pest" , "Tiny circus performer?" .)