Landmass - between seas, I assume (4)
I believe the answer is:
asia
'landmass' is the definition.
(Asia is an example)
'between seas i assume' is the wordplay.
'between' indicates a hidden word.
'ASIA' can be found hidden inside 'seas i assume'.
(Other definitions for asia that I've seen before include "Earth's largest continent" , "Put out" , "Major land mass" , "huge region" , "Large area" .)