Warning in advance (4)
I believe the answer is:
fore
This clue is a double definition.
'warning' is the first definition.
(shouted in golf)
'in advance?' is the second definition.
(thesaurus)
(Other definitions for fore that I've seen before include "Word of warning to golfers" , "Placed in front" , "A golfer's warning cry - ball in flight" , "At the bow" , "Warning cry on a golf course" .)