Silly John gets upset in school? (7)
I believe the answer is:
foolish
'silly' is the definition.
(similar in meaning)
'john gets upset in school?' is the wordplay.
'john' becomes 'loo' (loo is a kind of john).
'gets upset' shows that the letters should be reversed in order (I've seen 'getting upset' mean this).
'in' is an insertion indicator.
'school?' becomes 'fish'.
'loo' reversed gives 'ool'.
'ool' going inside 'fish' is 'FOOLISH'.
(Other definitions for foolish that I've seen before include "Silly, unwise" , "Imprudent" , "Silly, lacking common sense" , "Absurd, not sensible" , "Idiotic" .)