What a dog may do, the cur! (4)
I believe the answer is:
heel
'what a dog' is the definition.
(synonyms)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for heel that I've seen before include "Contemptible person" , "Command to a trained dog" , "3's weak point" , "Achilles' weak spot" , "Untrustworthy person" .)