Nine imperial gallons (of beer?) (6)

I believe the answer is:

firkin

(Other definitions for firkin that I've seen before include "Cooper's little effort" , "Brewing measure equal to 9 gallons" , "Nine-gallon wooden keg" , "Small cask for liquids, butter, fish" , "Small cask formerly used for liquids" .)

I've seen this clue in The Guardian.

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