Moving mass of ice (7)
I believe the answer is:
glacier
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">moving mass of ice</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)