Old boy imprisoned by new boy, lout (5)
I believe the answer is:
yobbo
'lout' is the definition.
(I've seen this before)
'old boy imprisoned by new boy' is the wordplay.
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
'imprisoned by' indicates putting letters inside.
'new' indicates anagramming the letters.
'boy' with letters rearranged gives 'ybo'.
'ob' put inside 'ybo' is 'YOBBO'.
(Other definitions for yobbo that I've seen before include "Surly youth" , "Young troublemaker" , "Lout" , "Rude and aggressive youth" , "Tough bloke" .)