Like one with a large landmass (4)
I believe the answer is:
asia
'large landmass' is the definition.
(I've seen this in another clue)
'like one with a' is the wordplay.
'like' becomes 'as'.
'one' becomes 'I' (Roman numeral).
'with' says to put letters next to each other.
'as'+'i'+'a'='ASIA'
(Other definitions for asia that I've seen before include "space for economic expansion?" , "It has most of the earth's population" , "Heavily populated area" , "Largest land-mass" , "huge amount of ground" .)