Unfilled part of a wine cask (6)
I believe the answer is:
ullage
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">unfilled part of a wine cask</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)