Threaten chaps with expert (6)
I believe the answer is:
menace
'threaten' is the definition.
(synonyms)
'chaps with expert' is the wordplay.
'chaps' becomes 'men' (chap can mean a man).
'with' means one lot of letters go next to another.
'expert' becomes 'ace' (ace can informally mean very good).
'men'+'ace'='MENACE'
(Other definitions for menace that I've seen before include "Intimidate" , "ominous atmosphere" , "serious nuisance" , "Threat or severe nuisance" , "Threaten with harm or danger" .)