Served from a keg (2,3)
I believe the answer is:
on tap
'served from a keg' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for on tap that I've seen before include "Always available" , "Immediately available like beer in pub" , "Ready to be drawn off, like beer" , "Ready for immediate use" , "where C or H is seen?" .)