Twice the normal-size wine bottle (6)
I believe the answer is:
magnum
'twice the normal-size wine bottle' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for magnum that I've seen before include "a lot of bottle!" , "that may be full of slugs" , "1.5 litre wine bottle" , "Two champers bottles in one" , "Double bubbly" .)