Large champagne bottle (6)
I believe the answer is:
magnum
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">large champagne bottle</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)