Narrow steep-sided valley (6)
I believe the answer is:
ravine
'narrow steep-sided valley' is the definition.
(I've seen this in another clue)
This is the entire clue.
(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" .)