Warmer - or cooler (8)
I believe the answer is:
radiator
I believe this clue is a double definition.
'warmer' is the first definition.
(radiator is a kind of warmer)
'cooler' is the second definition.
(I've seen this before)
'or' acts as a link.
(Other definitions for radiator that I've seen before include "Room warmer" , "Warmer - cooler" , "It spreads heat as in house" , "Aid or art, strangely, for the heater" , "Domestic heater" .)