Have they medicinal value at sea? (5)

I believe the answer is:

salts

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'have they' is the definition.
The definition suggests a plural noun which matches the answer.

I cannot understand how the rest of the clue works.

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(Other definitions for salts that I've seen before include "Six [EPSOM]'s might revive" , "Old sailors" , "See 25" , "Medicinal compounds; Epsom --" , "Epsom--; smelling --" .)

I've seen this clue in the Evening Standard.

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