Dog's pest (4)
I believe the answer is:
flea
'dog's pest' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for flea that I've seen before include "Blood-sucking parasites" , "little pest" , "In your ear, this insect is a rebuke" , "Circus insect?" , "Leaping insects" .)