Threaten people with card (6)
I believe the answer is:
menace
'threaten' is the definition.
(synonyms)
'people with card' is the wordplay.
'people' becomes 'men' (men are people).
'with' is a charade indicator (letters next to each other).
'card' becomes 'ace' (ace is a type of playing card).
'men'+'ace'='MENACE'
(Other definitions for menace that I've seen before include "Intimidation" , "ominous atmosphere" , "Act in a threatening manner" , "Threat; threaten" , "A source of danger" .)