Big wine bottle (6)
I believe the answer is:
magnum
'big wine bottle' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for magnum that I've seen before include "Double-sized wine bottle" , "Twice the normal-size wine bottle" , "lots of wine" , "Wine bottle twice normal size" , "that may be full of slugs" .)