At sea, chiefly? (2,3,4)
I believe the answer is:
in the main
I believe this clue is a double definition.
'at sea' is the first definition.
('the main' is an old name for the sea)
'chiefly?' is the second definition.
('in the main' can be a synonym of 'chiefly')
(Other definitions for in the main that I've seen before include "All things considered" , "Almost entirely" , "Generally - at sea?" , "seaborne?" , "usually" .)