Firmament (3)
I believe the answer is:
sky
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">firmament</span>' is the definition.<br/ ><span class="explanation-format__explanation">(sky is a kind of firmament)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for sky that I've seen before include "Heavens above!" , "Hit high into the air" , "Upper region, empyrean" , "Hit upwards" , "Outer space viewed from earth" .)