Ready to pour (5)
'ready to pour' 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 "Where to find a washer" , "Of liquid gas, ready to be drawn off" , "where C or H is seen?" , "Immediately available like beer in pub" , "Ready for immediate use" .)
(Another definition for ontap that I've seen is " readily available".)