Tap on a cask (6)
I believe the answer is:
spigot
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">tap on a cask</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)