Double-size wine bottle (6)
I believe the answer is:
magnum
'double-size wine bottle' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for magnum that I've seen before include "Arm" , "Two wine bottles in one" , "Large bottle of champagne, twice normal size" , "Just one bottle of champagne?" , "Double-size wine bottle" .)