Small barrel (3)
'small barrel' is the definition.
(I've seen this before)
This is all the 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" .)