A triad played on gold heater (8)
I believe the answer is:
radiator
'heater' is the definition.
(I know that radiator is a type of heater)
'a triad played on gold' is the wordplay.
'played' is an anagram indicator (letters playing around).
'on' means one lot of letters goes inside another (as in clothing 'on' a person).
'gold' becomes 'or' ('or' is the heraldic name for gold).
'triad' is an anagram of 'rdiat'.
'a' inserted inside 'rdiat' 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" .)