Doctor barks back (4)
I believe the answer is:
spay
'doctor' is the definition.
(I've seen this before)
'barks back' is the wordplay.
'barks' becomes 'yaps' (yapping is a kind of barking).
'back' says the letters should be written backwards.
'yaps' back-to-front is 'SPAY'.
(Other definitions for spay that I've seen before include "doctor" , "Neuter" , "Remove the ovaries of a female animal" , "Sterilise (a female animal)" , "Sterilize" .)