Barking pub rejected proof of age (5)
I believe the answer is:
rabid
'proof of age' is the definition.
Both the answer and definition are adjectives. Perhaps there's a link between them I don't understand?
'barking pub rejected' is the wordplay.
'barking' becomes 'di' (I can't justify this - if you can you should believe this answer much more).
'pub' becomes 'bar' (I've seen this before).
'rejected' says the letters should be written in reverse.
'di'+'bar'='dibar'
'dibar' reversed gives 'RABID'.
Can you help me to learn more?
(Other definitions for rabid that I've seen before include "Affected by 24 across" , "Ferocious" , "Fanatical, infected" , "Raging (having been bitten by a dog?)" , "Zealous" .)