Hilltop fire (6)
I believe the answer is:
beacon
'hilltop fire' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for beacon that I've seen before include "Warning, guiding, fire" , "Warning light on tower or hill" , "Warning fire or signal" , "A warning light as on headland" , "Signal fire - lighthouse" .)