Threat of star to workers (6)
I believe the answer is:
menace
'threat' is the definition.
(I know that menace is a type of threat)
'star to workers' is the wordplay.
'star' becomes 'ace' (synonyms).
'to' means one lot of letters go next to another (I've seen this in other clues).
'workers' becomes 'men' (men can mean staff or employees).
'ace' put after 'men' is 'MENACE'.
'of' acts as a link.
(Other definitions for menace that I've seen before include "A threatening quality" , "Harmful person" , "A source of danger" , "Serious threat" , "Intimidate" .)