Threaten people with a 'fine' (6)
I believe the answer is:
menace
'threaten' is the definition.
(synonyms)
'people with a fine' is the wordplay.
'people' becomes 'men' (men can refer to humanity as a whole).
'with' means one lot of letters go next to another.
'a fine' becomes 'ace' (I've seen this in other clues).
'men'+'ace'='MENACE'
(Other definitions for menace that I've seen before include "Threat or severe nuisance" , "A threatening quality" , "Intimidation" , "Intimidate" , "Threat, show of hostility" .)