Available from a keg (2,3)
I believe the answer is:
on tap
'available from a keg' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for on tap that I've seen before include "Ready for immediate use" , "Constantly available like beer from keg" , "where C or H is seen?" , "Available whenever needed" , "Immediately available like beer in pub" .)