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

I believe the answer is:

belay

I'm a little stuck... Click here to teach me more about this clue!

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

I don't understand the rest of the clue.

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.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.