Sound to cover expletive in unforgettable episode (5)
I believe the answer is:
bleep
'sound to cover expletive' is the definition.
'in unforgettable episode' is the wordplay.
'in' indicates the answer is hidden within the clue.
'BLEEP' can be found hidden inside 'unforgettable episode'.
(Other definitions for bleep that I've seen before include "Noise of alarm" , "Short high tone as a signal or warning" , "warning sound" , "High-pitched electrical signal" , "Electronic alarm sound" .)