Agreement in Berlin on port wine (5)
I believe the answer is:
'wine' is the definition.
(I know that Rioja is a type of red)
'agreement in berlin on port' is the wordplay.
'agreement in berlin' becomes 'ja' (I can't explain this - if you can you should believe this answer much more).
'on' is a charade indicator (letters next to each other).
'port' becomes 'rio' (Rio de Janeiro is a port).
'ja' put after 'rio' is 'RIOJA'.
Can you help me to learn more?
(Other definitions for rioja that I've seen before include "Spanish red wine" , "Spanish wine variety" , "Spanish drink" , "Spanish table wine" , "Popular Spanish wine" .)