Ready if needed (2,3)
I believe the answer is:
on tap
'ready if needed' 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" , "Ready to be drawn off, say beer" , "Freely available" , "Ready to be drawn off, like beer" , "Constantly available like beer from keg" .)