What dreadful lager contains is the dregs (6)
I believe the answer is:
ullage
'the dregs' is the definition.
(I've seen this before)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for ullage that I've seen before include "shortfall" , "The amount by which a bottle falls short of being full" , "dregs" , "Amount by which a liquid container falls short of being full" , "Loss by evaporation or leakage" .)