What the rotter said to the dog? (4)
I believe the answer is:
heel
'to the dog?' is the definition.
('heel' can be a synonym of 'dog')
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for heel that I've seen before include "Hindu loincloth" , "See 21" , "As a dog, obey" , "swine" , "Call to dog" .)