Crevasse (6)
I believe the answer is:
ravine
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">crevasse</span>' is the definition.<br/ ><span class="explanation-format__explanation">(thesaurus)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for ravine that I've seen before include "Canyon" , "'Deep, steep valley (6)'" , "Steep-sided gorge" , "Narrow, steep-sided valley" , "Deep narrow gorge with steep sides" .)