Crossword Genius

Shortage of drink in a vessel (6)

Ross

I believe the answer is:

ullage

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'shortage' is the definition.
(thesaurus)

I don't understand how the rest of the clue works.

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(Other definitions for ullage that I've seen before include "Amount by which a liquid container falls short of being full" , "dregs" , "Loss by evaporation or leakage" , "The amount by which a bottle falls short of being full" , "shortfall" .)

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