One invited to the pub (5,4)
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" .)