Ice sheet (4)
I believe the answer is:
floe
'ice sheet' is the definition.
(I've seen this in another clue)
This is the entire clue.
(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" .)