Sample white wine, swallowing first bit of Rioja (6)
I believe the answer is:
morsel
'sample' is the definition.
(thesaurus)
'white wine swallowing first bit of rioja' is the wordplay.
'white wine' becomes 'mosel' (type of white wine).
'swallowing' is an insertion indicator.
'first bit of rioja' becomes 'r' (1st letter of 'rioja').
'mosel' placed around 'r' is 'MORSEL'.
(Other definitions for morsel that I've seen before include "A tiny bit" , "Scrap, fragment" , "Tiny piece" , "Titbit" , "Crumb" .)