Like island with a large landmass (4)
I believe the answer is:
'large landmass' is the definition.
(I've seen this in another clue)
'like island with a' is the wordplay.
'like' becomes 'as'.
'island' becomes 'i' (geographical abbreviation).
'with' is a charade indicator (letters next to each other).
(Other definitions for asia that I've seen before include "Continent bordering three oceans" , "Populous continent" , "Heavily populated area" , "Large continent" , "huge amount of ground" .)