The main part of the earth (5)
I believe the answer is:
ocean
I believe this clue is a double definition.
'the main' is the first definition.
('ocean' can be a synonym of 'main')
'part of the earth' is the second definition.
('ocean' is part of 'earth')
(Other definitions for ocean that I've seen before include "An immense expanse of sea" , "Quantity of water" , "Sea - canoe (anag)" , "Vast sea" , "Big sea" .)