Agreement in Berlin on port wine (5)
I believe the answer is:
rioja
'wine' is the definition.
(Rioja is an example)
'agreement in berlin on port' is the wordplay.
'agreement in berlin' becomes 'ja' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'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'.
(Other definitions for rioja that I've seen before include "Spanish red wine" , "Spanish wine variety" , "Spanish drink" , "Spanish table wine" , "Popular Spanish wine" .)