Floating ice sheet (4)
I believe the answer is:
floe
'floating ice sheet' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for floe that I've seen before include "Floating flat mass of ice" , "Flat mass of sea ice" , "Piece of ice at sea" , "Sheet of ice floating in the sea" , "Part separated from a mass of ice" .)