Threaten people with an expert (6)
I believe the answer is:
menace
'threaten' is the definition.
(I know that threaten can be written as menace)
'people with an expert' is the wordplay.
'people' becomes 'men' (men are people).
'with' 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, show of hostility" , "Threaten with harm or danger" , "Threat; threaten" , "Harmful person" , "peril" .)