Ice sheet (4)
I believe the answer is:
floe
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">ice sheet</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for floe that I've seen before include "Separated piece of ice on water" , "A large lump of floating ice" , "Piece of ice at sea" , "Flat mass of sea ice" , "Frozen field" .)