Huge land mass of Sumatra is amazing northwards (4)
I believe the answer is:
asia
'huge land mass' is the definition.
(Asia is a continent)
'of sumatra is amazing northwards' is the wordplay.
'of' indicates a hidden word.
'northwards' says the letters should be written in reverse (in a down clue, letters go up or 'north').
'aisa' is hidden within 'sumatra is amazing'.
'aisa' written backwards gives 'ASIA'.
(Other definitions for asia that I've seen before include "Continent of the Hindu Kush" , "populous region" , "lands in the east" , "Heavily populated area" , "Continental land mass" .)