Sharp ridge (5)
I believe the answer is:
arete
'sharp ridge' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for arete that I've seen before include "part of mountain range" , "feature of range" , "Sharp ridge on mountain, with a tree" , "Narrow mountain ridge - eater (anag)" , "Glacial ridge" .)