Crossword Genius

An arresting thing for a sailor to say? (5)

Ross

I believe the answer is:

belay

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'say?' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for belay that I've seen before include "Turn a rope round something to secure it" , "Make fast" , "Stop" .)

I've seen this clue in the Evening Standard.
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