Junior tearaway, ultimately having no urge to reform (7)
I believe the answer is:
younger
'junior' is the definition.
(similar in meaning)
'tearaway ultimately having no urge to reform' is the wordplay.
'ultimately' indicates one should take the final letters.
'having' means one lot of letters go next to another.
'to reform' indicates an anagram.
The final letter of 'tearaway' is 'y'.
'no'+'urge'='nourge'
'nourge' is an anagram of 'ounger'.
'y'+'ounger'='YOUNGER'
(Other definitions for younger that I've seen before include "Not so old" , "More youthful" , "Comparatively raw" , "more boyish" , "Less aged" .)