Moving mass of ice (7)
I believe the answer is:
glacier
'moving mass of ice' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for glacier that I've seen before include "Slow-moving mass of ice" , "Creeping ice mass" , "one goes slowly downhill" , "One creeps downhill" , "Immense, slow-moving body of ice" .)