Threaten chaps with magic (6)
I believe the answer is:
menace
'threaten' is the definition.
(synonyms)
'chaps with magic' is the wordplay.
'chaps' becomes 'men' (chap means a man).
'with' means one lot of letters go next to another.
'magic' becomes 'ace' (both can mean very good).
'men'+'ace'='MENACE'
(Other definitions for menace that I've seen before include "Threaten with violence" , "Threat or severe nuisance" , "Threat; threaten" , "Act in a threatening manner" , "Intimidate" .)