Pest on a pooch (4)
I believe the answer is:
flea
'pest on a pooch' 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 "one is proverbially fit" , "Small jumper in small circus" , "tiny jumper" , "Bloodsucking parasite" , "Jumping bloodsucker" .)