Primary sea (4)
I believe the answer is:
main
I believe this is a double definition.
'primary' is the first definition.
(I know that primary can be written as main)
'sea' is the second definition.
(synonyms)
(Other definitions for main that I've seen before include "Source of electricity" , "Salt water" , "river in Germany" , "Chief; sea; pipe" , "Cardinal" .)