This sign means nothing to workers (4)
I believe the answer is:
omen
'this sign' is the definition.
(omen is a kind of sign)
'nothing to workers' is the wordplay.
'nothing' becomes 'O' (looks like zero - 0).
'to workers' becomes 'men' (men can mean staff or employees).
'o'+'men'='OMEN'
'means' is the link.
(Other definitions for omen that I've seen before include "Harbinger, portent" , "Sign of something to come" , "Warning - sign" , "Portent, prophetic sign" , "Portent, prophetic indicator" .)