Where C or H may be readily available (2,3)
I believe the answer is:
on tap
'readily available' is the definition.
(I've seen this in another clue)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for on tap that I've seen before include "Freely available" , "Constantly available like beer from keg" , "Available whenever needed" , "Ready to be drawn off, say beer" , "kept in a cask" .)