Threaten workers with one (6)
I believe the answer is:
menace
'threaten' is the definition.
(synonyms)
'workers with one' is the wordplay.
'workers' becomes 'men' (men can mean staff or employees).
'with' means one lot of letters go next to another.
'one' becomes 'ace' (ace has a value of one in some card games).
'men'+'ace'='MENACE'
(Other definitions for menace that I've seen before include "serious nuisance" , "Threat or severe nuisance" , "A threatening quality" , "Serious threat" , "Intimidation" .)