Made mistake in discussion before everything's gone belly up — pantomime! (10)
I believe the answer is:
cinderella
'pantomime' is the definition.
(I've seen this before)
'made mistake in discussion before everything's gone belly up' is the wordplay.
I cannot quite see how this works, but
'in' is found in the answer.
'before' could be 'ere' (usually literary) and 'ere' is present in the answer.
This explanation may well be incorrect...
(Other definitions for cinderella that I've seen before include "a success in the end?" , "8 [BALLET]" , "Scullery maid" , "A fabulous heroine" , "Fairy tale heroine" .)