A triad played on gold heater (8)
I believe the answer is:
radiator
'heater' is the definition.
(radiator is a kind of heater)
'a triad played on gold' is the wordplay.
'played' indicates an anagram.
'on' is an insertion indicator (as in clothing \'on\' a person).
'gold' becomes 'or' ('or' is the heraldic name for gold).
'triad' anagrammed gives 'radit'.
'a' inserted within 'radit' is 'radiat'.
'radiat'+'or'='RADIATOR'
(Other definitions for radiator that I've seen before include "Aid or art, strangely, for the heater" , "Heater -- or cooler!" , "Its content is wet!" , "Domestic heating device" , "Domestic heat source" .)