Fissure, chasm (4)
I believe the answer is:
rift
<span class="explanation-format__ClueExplanation">I believe this clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">fissure</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I know that rift is a type of fissure)</span><br/ ><br/ >'<span class="explanation-format__original">chasm</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ ></span>
(Other definitions for rift that I've seen before include "gap" , "Breach - falling-out" , "Serious falling out" , "Breach or quarrel" , "Cleft or fissure" .)