Open, as a keg (3)
I believe the answer is:
tap
'open as a keg' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for tap that I've seen before include "that's less hard than 11 [BASH] or 22 [BATTER]" , "water supply" , "Intercept; light knock" , "Somewhere to drink beer" , "Flow controller; knock" .)