To the workers, one is a threat (6)
I believe the answer is:
menace
'threat' is the definition.
(I know that threat can be written as menace)
'to the workers one' is the wordplay.
'to the workers' becomes 'men' (men can mean staff or employees).
'one' becomes 'ace' (ace has a value of one in some card games).
'men'+'ace'='MENACE'
'is a' acts as a link.
(Other definitions for menace that I've seen before include "Threat or severe nuisance" , "Intimidation" , "ominous atmosphere" , "Threaten with harm or danger" , "Threaten with violence" .)