Drink when in Bordeaux's dear (6)
I believe the answer is:
chaser
'drink' is the definition.
(as in a shot and chaser)
'when in bordeaux's dear' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'in' is an insertion indicator.
'bordeaux's dear' becomes 'cher' ('dear' in French - Bordeaux is in France).
'as' put within 'cher' is 'CHASER'.
(Other definitions for chaser that I've seen before include "Different kind of drink - horse" , "Drink following another of a different kind" , "Pursuit cannon; one engraving" , "Drink that follows immediately after another one" , "second drink" .)