Continents considered as a whole one in ruined US area (7)
I believe the answer is:
eurasia
'continents considered as a whole' is the definition.
(Europe and Asia)
'one in ruined us area' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' means one lot of letters goes inside another.
'ruined' indicates anagramming the letters.
'us'+'area'='usarea'
'usarea' anagrammed gives 'eurasa'.
'i' put within 'eurasa' is 'EURASIA'.
(Other definitions for eurasia that I've seen before include "Turkey, and so on" , "Landmass formed by two continents" , "Large northern hemisphere land mass" , "large part of world" , "Landmass extending from Cape Roca in the west to Cape Dezhnev in the east" .)