Crossword Genius

Main geographical feature? (5)

Ross

I believe the answer is:

ocean

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This clue could be a double definition.

'main' is the first definition.
(synonyms)

'geographical feature?' is the second definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.

Can you help me to learn more?

(Other definitions for ocean that I've seen before include "It's pretty wet!" , "Vast sea" , "Sea - canoe (anag)" , "Salt water expanse" , "Large area of water" .)

I've seen this clue in The Irish Independent.
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