Streamlined mound formed by glacial action (7)
I believe the answer is:
drumlin
(Other definitions for drumlin that I've seen before include "Small glacier-formed hill" , "Small elongated oval hill, in Cavan, say" , "Hummock of boulder clay left by a retreating glacier" , "'Small ridge, common in Cavan and Monaghan (7)'" , "oval mound" .)