Omelette’s wrong — one sent back everything (8)
I believe the answer is:
tortilla
'omelette's' is the definition.
(I've seen this before)
'wrong one sent back everything' is the wordplay.
'wrong' becomes 'tort' (legal term**).
'one' becomes 'i' (Roman numeral).
'sent back' says the letters should be written in reverse.
'everything' becomes 'all' (I've seen this before).
'all' in reverse letter order is 'lla'.
'tort'+'i'+'lla'='TORTILLA'
(Other definitions for tortilla that I've seen before include "Cake including maize" , "In which one may see salsa" , "Mexican corn meal pancake" , "Mexican 7 across" , "Spanish omelette" .)