A portent for workers joining circle (4)
I believe the answer is:
omen
'a portent' is the definition.
('omen' can be a synonym of 'portent')
'workers joining circle' is the wordplay.
'workers' becomes 'men' (men can mean staff or employees).
'joining' is a charade indicator (letters next to each other).
'circle' becomes 'O' (O is a circle).
'men' put after 'o' is 'OMEN'.
'for' is the link.
(Other definitions for omen that I've seen before include "Sign of likely future event" , "Sign of a future event, good or bad" , "Gloomy sign" , "Warning - sign" , "Sign of something to come" .)