One acting badly about food sent back Big Mac? (9)
I believe the answer is:
hamburger
'big mac?' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
'one acting badly about food sent back' is the wordplay.
'one acting badly' becomes 'ham' (I can't explain this - if you can you should give a lot more credence to this answer).
'about' becomes 're' (regarding).
'food' becomes 'grub' (grub is a kind of food).
'sent back' says the letters should be written backwards.
're'+'grub'='regrub'
'regrub' in reverse letter order is 'burger'.
'ham'+'burger'='HAMBURGER'
(Other definitions for hamburger that I've seen before include "Citizen of Europe" , "beefcake" , "Staple of the fast food industry" , "minced beef roll" , "Item of fast food" .)