Lights-out tune (4)
I believe the answer is:
taps
'lights-out tune' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for taps that I've seen before include "pierces" , "Military signal and faucets" , "Knocks gently" , "Strikes lightly; eavesdropping devices" , "Uses listening devices" .)