Deep opening (5)
I believe the answer is:
chasm
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">deep opening</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for chasm that I've seen before include "Gaping hollow" , "Yawning gap" , "Deep cleft in ground" , "A deep opening in the earth" , "One yawns" .)