What a dog might do for a cad (4)
I believe the answer is:
heel
'for a cad' is the definition.
(synonyms)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for heel that I've seen before include "Back part of the foot" , "Command to a trained dog" , "Come here, Rover!" , "Hindu loincloth" , "Untrustworthy person" .)