School rogue in casualty? Gosh! (7)
I believe the answer is:
academy
'school' is the definition.
(I know that academy is a type of school)
'rogue in casualty? gosh' is the wordplay.
'rogue' becomes 'cad' (I've seen this before).
'in' means one lot of letters goes inside another.
'casualty?' becomes 'ae' (abbreviation for Accident and Emergency).
'gosh' becomes 'my' (both are old-fashioned expressions of surprise).
'cad' going within 'ae' is 'acade'.
'acade'+'my'='ACADEMY'
(Other definitions for academy that I've seen before include "Learning institution" , "Secondary school" , "Teaching establishment" , "School for specialised education" , "Training school" .)