Word of caution (6)
I believe the answer is:
caveat
'word of caution' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for caveat that I've seen before include "Vacate (anag) - warning" , "Warning, eg to buyer" , "'Caution, warning (6)'" , "Warning (of conditions)" , "At cave (anag.)" .)