Made mistake in discussion before everything's gone belly up — pantomime! (10)
'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 "One whose talent is overlooked", "A fabulous heroine", "[FAIRY STORY REF]", "Fairy tale heroine", "Scullery maid".)