Dieter's agreement on port wine (5)
I believe the answer is:
rioja
'wine' is the definition.
(Rioja is an example)
'dieter's agreement on port' is the wordplay.
'dieter's agreement' becomes 'ja' ('yes' in German; Dieter is a German name).
'on' is a charade indicator (letters next to each other).
'port' becomes 'rio' (I've seen this before).
'ja' after 'rio' is 'RIOJA'.
(Other definitions for rioja that I've seen before include "it comes from grapevine" , "Popular Spanish wine" , "kind of red" , "Dry red wine from Spain" , "Spanish drink" .)