EU cadet naughty in school (7)
I believe the answer is:
educate
'school' is the definition.
(synonyms)
'eu cadet naughty' is the wordplay.
'naughty' indicates an anagram.
'eu'+'cadet'='eucadet'
'eucadet' is an anagram of 'EDUCATE'.
'in' acts as a link.
(Other definitions for educate that I've seen before include "Impart knowledge" , "Teach - instruct" , "Enlighten" , "Teach - school" , "what teacher should do, to be effective" .)