Flop Gilbert and Sullivan initially backed (3)
I believe the answer is:
'flop' is the definition.
'gilbert and sullivan initially backed' is the wordplay.
'initially' indicates taking the first letters.
'backed' is a reversal indicator.
The first letters of 'gilbert and sullivan' is 'gas'.
'gas' back-to-front is 'SAG'.
(Other definitions for sag that I've seen before include "Lose energy" , "Slump" , "Sink, hang down" , "Sinking" , "Droop, sink in the middle" .)