Old boy imprisoned by silly boy, lout (5)
I believe the answer is:
yobbo
'lout' is the definition.
(I've seen this before)
'old boy imprisoned by silly boy' is the wordplay.
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
'imprisoned by' means one lot of letters goes inside another.
'silly' indicates an anagram.
'boy' anagrammed gives 'ybo'.
'ob' inserted inside 'ybo' is 'YOBBO'.
(Other definitions for yobbo that I've seen before include "Thug" , "Rowdy" , "Rude and aggressive youth" , "Arrive" , "Lout" .)