Drifting ice (4)
I believe the answer is:
floe
'drifting ice' 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 "Comparatively small piece of floating ice" , "Separated piece of ice on water" , "Flat mass of sea ice" , "Piece of ice at sea" , "A field of floating ice" .)