Choosing French wine, discovering lots inside — gallons! (6)
I believe the answer is:
voting
'choosing' is the definition.
(voting is a kind of choosing)
'french wine discovering lots inside gallons' is the wordplay.
'french wine' becomes 'vin' ('wine' in French).
'discovering' means to look at the middle letters (I've seen 'discovered' mean this (dis-covered - outer letters removed)).
'inside' means one lot of letters goes inside another.
'gallons' becomes 'g' (abbreviation).
The central letters of 'lots' are 'ot'.
'vin' placed around 'ot' is 'votin'.
'votin'+'g'='VOTING'
(Other definitions for voting that I've seen before include "making one cross?" , "polling process" , "Selecting by ballot" , "Indicating choice or opinion, in an election or debate" , "Taking part in poll" .)