Very deep chasm (5)
I believe the answer is:
abyss
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">very deep chasm</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 abyss that I've seen before include "Deep hole, seems bottomless" , "Fathomless pit" , "Deep gulf" , "Bottomless pit" , "Measureless chasm" .)