Large champagne bottle (6)
I believe the answer is:
magnum
'large champagne bottle' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for magnum that I've seen before include "Two wine bottles in one" , "Twice the normal-size wine bottle" , "Double-size wine bottle" , "that may be full of slugs" , "Two champers bottles in one" .)