Regularly taking milk kept in cask (2,3)
I believe the answer is:
on tap
'kept in cask' is the definition.
'regularly taking milk' is the wordplay.
'regularly taking' becomes 'on' (as in 'on drugs').
'milk' becomes 'tap' (both can mean to exploit or use).
'on'+'tap'='ON TAP'
(Other definitions for on tap that I've seen before include "readily accessible" , "Available (for drinking?)" , "Ready for immediate use" , "Ready to be drawn off, say beer" , "kept in a cask" .)