Troublemaker, youth who ultimately reformed in gaol (8)
I believe the answer is:
hooligan
'troublemaker' is the definition.
(I've seen this before)
'youth who ultimately reformed in gaol' is the wordplay.
'ultimately' says to take the final letters.
'reformed' indicates an anagram.
The final letters of 'youth who' are 'ho'.
'in'+'gaol'='ingaol'
'ingaol' anagrammed gives 'oligan'.
'ho'+'oligan'='HOOLIGAN'
(Other definitions for hooligan that I've seen before include "Violent young troublemaker" , "Lawless type" , "Wild ruffian" , "Undesirable" , "Destructive or badly-behaved youth" .)