By and large at sea? (2,3,4)
I believe the answer is:
in the main
I believe this clue is a double definition.
'by and large' is the first definition.
(thesaurus)
'at sea?' is the second definition.
('the main' is an old name for the sea)
(Other definitions for in the main that I've seen before include "All things considered" , "Swimming" , "Almost entirely" , "Where piped water collects" , "For the large part" .)