The old devil! (5)
I believe the answer is:
harry
'devil' is the definition.
(thesaurus)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for harry that I've seen before include "See 20" , "Wizard Potter" , "Persistently attack (an enemy)" , "Prince William's younger brother" , "Enfield, perhaps" .)