Greenhouse effect? (4)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">greenhouse effect?</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for heat that I've seen before include "Toast" , "Level of spice" , "Preliminary round in a contest" , "Warm up" , "Hotness" .)