A sailor in goal (6)

I believe the answer is:

seaman

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'a sailor' is the definition.
(I know that seaman is a type of sailor)

I cannot understand how the rest of the clue works.

(Other definitions for seaman that I've seen before include "Mariner" , "crew member" , "A manse (anag.)" , "Sailor" .)

I've seen this clue in the Evening Standard.

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