Beer barrel (6)
'beer 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 "What's found at King's Head" , "Small cask or barrel" , "Sort of beer" , "Barrel as for beer" , "Small beer cask" .)
(Other definitions for firkin that I've seen before include "Small wooden barrel - nine gallons" , "Cooper's little effort" , "Brewing measure of nine gallons" , "Wooden keg with 9 imperial gallons" , "Small cask for liquids, butter, fish" .)