A word of warning (6)
I believe the answer is:
beware
'a word of warning' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for beware that I've seen before include "Archaic warning to watch out, danger ahead" , "Be alert for dangers" , "Take heed" , "Have caution, be on guard" , "Be on one's guard" .)