Stale ingredient of school dinners (3)
I believe the answer is:
old
'stale' is the definition.
('old' can be a synonym of 'stale')
'ingredient of school dinners' is the wordplay.
'ingredient of' indicates a hidden word.
'OLD' is hidden within 'school dinners'.
(Other definitions for old that I've seen before include "Senescent" , "Former, previous" , "Long in the tooth" , "out of the ark" , "Wild clematis" .)