Radiator, say (6)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">radiator say</span>' is the definition.<br/ ><span class="explanation-format__explanation">(radiator is a kind of heater)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for heater that I've seen before include "Room warmer" , "Source of warmth; gun" , "Radiator" , "fire" , "Warming device" .)