At first reticent — is outgoing just after wine? (5)
I believe the answer is:
rioja
'wine?' is the definition.
(Rioja is an example)
'at first reticent is outgoing just after' is the wordplay.
'at first' indicates taking the first letters.
The initial letters of 'reticent is outgoing just after' is 'RIOJA'.
(Other definitions for rioja that I've seen before include "Spanish wine variety" , "Spanish table wine" , "Spanish drink" , "Open space (ancient Greek)" , "Wine from northern Spain" .)