This is a double definition - the clue gives two different meanings of the same word, with no linking word like 'and' between them.
Look for a word that means both 'large quantity of beer' and relates to the phrase about being powerless. The phrase 'over which one is powerless' hints at a common idiom where someone has you in a position of no control.
Answer: BARREL | Double definition - a barrel is a large quantity of beer, and if someone has you 'over a barrel' then you're powerless