Unfilled part of a wine cask (6)
I believe the answer is:
ullage
'unfilled part of a wine cask' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ullage that I've seen before include "shortfall" , "dregs" , "Amount by which a liquid container falls short of being full" , "Loss by evaporation or leakage" , "The amount by which a bottle falls short of being full" .)