Pest on a pet (4)
I believe the answer is:
flea
'pest on a pet' 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 "Jumping, blood-sucking insect" , "Biting type" , "Bloodsucking parasite" , "source of infestation" , "Little jumping insect" .)