Threaten chaps with expert (6)
I believe the answer is:
menace
'threaten' is the definition.
('menace' can be a synonym of 'threaten')
'chaps with expert' is the wordplay.
'chaps' becomes 'men' (I've seen this before).
'with' is a charade indicator (letters next to each other).
'expert' becomes 'ace' (ace is a kind of expert).
'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" .)