Crossword Genius

Main reason to get excited — Reagan's quit (3)

Ross

I believe the answer is:

sea

'main' is the definition.
(synonyms)

I don't understand how the rest of the clue works.

(Other definitions for sea that I've seen before include "The tide" , "Ocean area" , "that's easily reached from Dover?" , "It's stocked with fish" , "Large body of salt water" .)

I've seen this clue in The Guardian.
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