Slow-moving masses of ice (8)
I believe the answer is:
glaciers
'slow-moving masses of ice' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for glaciers that I've seen before include "Slowly moving ice sheets" , "Ice flows" , "Slow-moving ice rivers" , "Slow moving masses of ice" , "Slow-moving masses of ice" .)