Principal ocean (4)
I believe the answer is:
main
I believe this is a double definition.
'principal' is the first definition.
(both can mean major)
'ocean' is the second definition.
(synonyms)
(Other definitions for main that I've seen before include "Might on the high seas" , "location for 9 [ARMADA]" , "Most important or greatest in size" , "(The mighty) sea" , "Source of water" .)