Be on one's guard (6)
I believe the answer is:
beware
'be on one's guard' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for beware that I've seen before include "Archaic word for look out, mind yourself" , "cave" , "Archaic injunction to be careful, look out" , "Take heed" , "Wee bra (anag)" .)