Heat unit (7)
I believe the answer is:
calorie
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">heat unit</span>' is the definition.<br/ ><span class="explanation-format__explanation">(calorie is a kind of heat unit)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for calorie that I've seen before include "A recoil (anag.)" , "Food energy unit" , "Unit of energy in foods" , "generating a bit of heat" , "Unit of heat energy" .)