"Tap!", quartet at Harden said (6)
I believe the answer is:
faucet
'tap' is the definition.
(term for a tap in US English)
I cannot understand how the rest of the clue works.
(Other definitions for faucet that I've seen before include "An American water tap" , "North American tap" , "Pipe" , "Spigot, tap" , "One may turn on this" .)