One very upset about commercial describing food prepared beforehand (4-5)
I believe the answer is:
oven-ready
'describing food prepared beforehand' is the definition.
(as in food ready for the oven)
'one very upset about commercial' is the wordplay.
'upset' indicates anagramming the letters.
'about' is an insertion indicator.
'commercial' becomes 'ad' (abbreviation for advertisement).
'one'+'very'='onevery'
'onevery' with letters rearranged gives 'ovenrey'.
'ovenrey' enclosing 'ad' is 'OVEN-READY'.
(Other definitions for oven-ready that I've seen before include "Already prepared for cooking (of chickens)" , "Pre-cooked" .)