A French-American cold mushroom meal is not on (12)
I believe the answer is:
unacceptable
'is not on' is the definition.
(I've seen this before)
'a french-american cold mushroom meal' is the wordplay.
'a french' becomes 'un' ('a' in French).
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'cold' becomes 'c' (eg on taps).
'mushroom' becomes 'cep' (Boletus edulis - an edible mushroom).
'meal' becomes 'table'.
'un'+'a'+'c'+'cep'+'table'='UNACCEPTABLE'
(Other definitions for unacceptable that I've seen before include "Not satisfactory" , "Not allowable" .)