Regressive boy wrecked last old reformatory (7)
I believe the answer is:
borstal
'old reformatory' is the definition.
(I've seen this before)
'regressive boy wrecked last' is the wordplay.
'regressive' shows that the letters should be reversed in order (to regress is to go backwards).
'boy' becomes 'rob' (boy's name).
'wrecked' indicates an anagram.
'rob' written backwards gives 'bor'.
'last' with letters rearranged gives 'stal'.
'bor'+'stal'='BORSTAL'
(Other definitions for borstal that I've seen before include "Former reform school" , "former institution" , "Reformatory school" , "Penal institution" , "Former establishment for offenders aged 15 to 21" .)