Sailor's treaty (4)

I believe the answer is:

salt

This clue is a double definition.

'sailor's' is the first definition.
(salt is a kind of sailor)

'treaty' is the second definition.
(I know that SALT is a treaty)

(Other definitions for salt that I've seen before include "Sea dog" , "Preservative" , "Common substance" , "Crystalline substance used to season food" , "Seasoned sailor" .)

I've seen this clue in The Irish Independent.

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