Narrow steep-sided valley (6)
I believe the answer is:
ravine
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">narrow steep-sided valley</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for ravine that I've seen before include "Vainer in narrow gorge" , "Deep narrow gorge between mountains" , "Defile" , "Narrow, steep-sided valley" , "Canyon" .)