Sea arch (4)

I believe the answer is:

main

I believe this 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" .)

I've seen this clue in The Independent.

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