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