Gravelly ridge (5)
I believe the answer is:
esker
'gravelly ridge' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for esker that I've seen before include "gravelly deposit" , "Sediment" , "bar, perhaps" , "Glacial ridge" , "Gravel ridge; reeks (anag.)" .)