Crack, split (4)
I believe the answer is:
rift
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">crack</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I know that rift is a type of crack)</span><br/ ><br/ >'<span class="explanation-format__original">split</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this in other clues)</span><br/ ><br/ ></span>
(Other definitions for rift that I've seen before include "Serious falling out" , "Breach - falling-out" , "A disagreement or an earth cleft" , "Crack, split, break" , "Break in relations; type of valley" .)