Warning a worker is about to fight (6)
I believe the answer is:
beware
'warning' is the definition.
('beware!' might be shouted as a warning)
'a worker is about to fight' is the wordplay.
'a worker' becomes 'bee' (a worker bee).
'is about' means one lot of letters goes inside another.
'to fight' becomes 'war' (warring is a kind of fighting).
'bee' enclosing 'war' is 'BEWARE'.
(Other definitions for beware that I've seen before include "Look out, take care" , "Archaic word for look out, mind yourself" , "Archaic injunction to be careful, look out" , "Cry of warning" , "Archaic warning to watch out, danger ahead" .)