Bolshy cook cut short misbehaving pupil (9)
I believe the answer is:
schoolboy
'pupil' is the definition.
(schoolboy is a kind of pupil)
'bolshy cook cut short misbehaving' is the wordplay.
'cut short' means to remove the last letter.
'misbehaving' indicates an anagram.
'bolshy'+'cook'='bolshycook'
'bolshycook' with its final letter removed is 'bolshycoo'.
'bolshycoo' anagrammed gives 'SCHOOLBOY'.
(Other definitions for schoolboy that I've seen before include "Flashman, for example?" , "Pupil at Eton, say" , "one of man's 22 20 [SEVEN AGES]" , "Lad in education" , "One studying" .)