When going to war, is not linked with a friend (4)

I believe the answer is:

ship

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

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for ship that I've seen before include "Ocean vessel" , "Large boat, dispatch" , "Convey (goods); a vessel" , "craft of 1d [ADVENTURE]" , "See 18" .)

I've seen this clue in the Evening Standard.

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