Warming device (8)
'warming device' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for heater that I've seen before include "Warming device" , "Source of warmth; gun" , "Room warmer" , "Radiator" , "fire" .)
(Other definitions for radiator that I've seen before include "House-warmer; engine-cooler" , "Apparatus for heating" , "It distributes heat in home, filled with hot water" , "Aid or art, strangely, for the heater" , "Domestic heat source" .)