Shameful rebel is addict on the rampage (13)
I believe the answer is:
discreditable
'shameful' is the definition.
(I've seen this before)
'rebel is addict on the rampage' is the wordplay.
'on the rampage' is an anagram indicator.
'rebel'+'is'+'addict'='rebelisaddict'
'rebelisaddict' is an anagram of 'DISCREDITABLE'.
(Other definitions for discreditable that I've seen before include "Shameful" , "Tending to bring harm to a reputation" .)