Small barrel (3)
'small barrel' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for keg that I've seen before include "Small cask or barrel" , "What's found at King's Head" , "Small beer cask" , "Sort of beer" , "Barrel as for beer" .)
(Other definitions for firkin that I've seen before include "Brewing measure equal to 9 gallons" , "Nine-gallon wooden keg" , "Small cask formerly used for liquids" , "Wooden keg with 9 imperial gallons" , "Brewing measure of nine gallons" .)