19 across
Large
quantity
of
beer
over
which
one
is
powerless
(6)
Large quantity of beer over which one is powerless (6)
hints
0
checks
0
hints
The whole clue is the definition (& lit or double definition).
How the clue works
The definition is large quantity of beer — a barrel being your classic cask measure. Then over which one is powerless nudges you towards the idiom "over a barrel", meaning completely helpless. Both halves are pointing at the same answer independently, so there's no separate wordplay to unpick here.