Moving ice mass (7)
I believe the answer is:
glacier
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">moving ice mass</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for glacier that I've seen before include "One creeps downhill" , "Creeping ice mass" , "Mass of ice moving down a mountain" , "Slow-moving mass of ice" , "Slow-moving flow of ice" .)