Enemy circles large ice sheet (4)
I believe the answer is:
floe
'ice sheet' is the definition.
(thesaurus)
'enemy circles large' is the wordplay.
'enemy' becomes 'foe' (I've seen this before).
'circles' means one lot of letters goes inside another.
'large' becomes 'l'.
'foe' going around 'l' is 'FLOE'.
(Other definitions for floe that I've seen before include "A large lump of floating ice" , "Sheet of ice floating in the sea" , "Frozen field" , "Comparatively small piece of floating ice" , "Floating flat mass of ice" .)