Tap on a cask (6)
I believe the answer is:
spigot
'tap on a cask' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for spigot that I've seen before include "Peg to control the flow of a tap" , "Tap - stopper" , "Peg for a cask's vent" , "Tap for barrel" , "Vent hole peg - US tap" .)