Fall flat (4)
'fall flat' is the definition.
(synonyms)
This is all the clue.
(Other definitions for flop that I've seen before include "Failure - loll" , "Collapse loosely - total failure" , "Fall clumsily, liked failed play" , "Total failure; fall heavily" , "No success" .)
(Other definitions for bomb that I've seen before include "Drop" , "not to be hit!" , "Exploding device" , "Explosive weapon" , "Flop - load of money" .)