Champagne stopper (4)
I believe the answer is:
cork
'champagne stopper' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for cork that I've seen before include "Stopper - Irish port" , "Wine bottle plug" , "Irish port - something in a wine bottle?" , "Common bottle stopper" , "Buoyant substance from a tree" .)