Worker about to fight - watch out (6)
I believe the answer is:
'watch out' is the definition.
(bewaring is a kind of watching out)
'worker about to fight' is the wordplay.
'worker' becomes 'bee' (I've seen this before).
'about' is an insertion indicator.
'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 "Archaic word for look out, mind yourself" , "Look out, take care" , "Take heed" , "Steer clear of" , "Archaic warning to watch out, danger ahead" .)