A French port -- open a bottle (6)
I believe the answer is:
uncork
'open a bottle' is the definition.
(I've seen this before)
'a french port' is the wordplay.
'a french' becomes 'un' ('a' in French).
'port' becomes 'cork' (I've seen this in other clues**).
'un'+'cork'='UNCORK'
(Other definitions for uncork that I've seen before include "Open (Chablis, say)" , "Remove traditional stopper from wine bottle" , "Open (a wine bottle)" , "Release the stopper" , "Clear a bottleneck" .)