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