In which to transport quality champagne? (7,3)
I believe the answer is:
vintage car
'quality champagne?' is the definition.
The definition suggests a singular noun which matches the answer.
'in which to transport' is the wordplay.
'in which' becomes 'vintage' (I am not sure about this - if you are sure you should believe this answer much more).
'to transport' becomes 'car' (car is a kind of transport).
'vintage'+'car'='VINTAGE CAR'
(Other definitions for vintage car that I've seen before include "Old wheels" , "one for Brighton rally?" , "Transport once" , "Automobile built before 1930" , "old vehicle" .)