Sea arch (4)
I believe the answer is:
main
This clue is a double definition.
'sea' is the first definition.
(synonyms)
'arch' is the second definition.
(thesaurus)
(Other definitions for main that I've seen before include "Chief; sea; pipe" , "source of gas" , "Principal or high seas" , "associated with the Spanish?" , "Chief, primary" .)