Large wine bottle (8)
'large 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 "Wine bottle twice normal size" , "that may be full of slugs" , "One point five litre wine bottle" , "1.5 litre wine bottle" , "Just one bottle of champagne?" .)
'large wine bottle' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for jeroboam that I've seen before include "'Large wine bottle, equal to four standard ones (8)'" , "Very large wine bottle named after old king of Israel" , "Four-times bottle size" , "Wine bottle 4-12 times the ordinary size" , "Very large wine bottle named after Israeli king" .)