Setters? Mean scoundrels! (4)
I believe the answer is:
dogs
'setters?' is the definition.
(setter is a kind of dog)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for dogs that I've seen before include "pets" , "eg 27 terriers" , "Canines" , "Setters, for example" , "Hounds" .)