Crossword Genius

One invited to the pub (5,4)

Ross

I believe the answer is:

guest beer

'one' is the definition.
Both the answer and definition are singular nouns.
Perhaps they are linked in a way I don't understand?

I cannot understand how the remainder of the clue works.

(Other definitions for guest beer that I've seen before include "Not a pint of the usual" , "Temporarily-available tipple" , "Pub's irregular offering" .)

I've seen this clue in The Guardian.
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