Like draft beer (5)
'like draft beer' 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 "kept in a cask" , "Ready to be drawn off, like beer" , "handy" , "Where you may see H or C" , "Ready for immediate use" .)
(Another definition for ontap that I've seen is " readily available".)