Freely available (2,3)
I believe the answer is:
'freely available' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for on tap that I've seen before include "Where to find a washer" , "Constantly available like beer from keg" , "kept in a cask" , "Available whenever needed" , "Ready to be drawn off, say beer" .)