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' says to put 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" .)