Available as the letter H or C (2,3)
I believe the answer is:
on tap
'available' is the definition.
(similar in meaning)
'the letter h or c' is the wordplay.
I cannot really see how this works, but
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is located in the answer.
'letter' could be 'p' and 'p' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
'as' acts as a link.
Can you help me to learn more?
(Other definitions for on tap that I've seen before include "Ready to be drawn off, say beer" , "Always available" , "Available (for drinking?)" , "Constantly available like beer from keg" , "where C or H is seen?" .)