To threaten people takes an expert (6)
I believe the answer is:
menace
'to threaten' is the definition.
(synonyms)
'people takes an expert' is the wordplay.
'people' becomes 'men' (men are people).
'takes' is a charade indicator (letters next to each other).
'an expert' becomes 'ace' (ace can informally mean very good).
'men'+'ace'='MENACE'
(Other definitions for menace that I've seen before include "Threat, danger" , "Threaten with harm or danger" , "ominous atmosphere" , "Threat; threaten" , "Harmful person" .)