Predominant sea (4)
I believe the answer is:
main
I believe this is a double definition.
'predominant' is the first definition.
(main can mean predominant or principal)
'sea' is the second definition.
(synonyms)
(Other definitions for main that I've seen before include "river in Germany" , "associated with the Spanish?" , "source of gas" , "Ocean or principal pipe" , "Foremost" .)