It's filled with melted cheese, as is Antarctica (3,5)
I believe the answer is:
ice sheet
'as is antarctica' is the definition.
(as in the Antarctic ice sheet)
'it's filled with melted cheese' is the wordplay.
'it's' becomes 'it' (I've seen this in other clues).
'filled with' means one lot of letters goes inside another.
'melted' indicates an anagram.
'cheese' anagrammed gives 'ceshee'.
'it' enclosing 'ceshee' is 'ICE SHEET'.
(Other definitions for ice sheet that I've seen before include "Extensive frozen layer over land" , "white surface layer" , "Extensive frozen area" .)