The dope fiend did reform, in turn (6)
I believe the answer is:
addict
'the dope fiend' is the definition.
'did reform in turn' is the wordplay.
'reform' is an anagram indicator.
'in' indicates putting letters inside.
'turn' becomes 'act' (theatre**).
'did' with letters rearranged gives 'ddi'.
'ddi' put inside 'act' is 'ADDICT'.
(Other definitions for addict that I've seen before include "Devoted fan" , "fiend" , "One dependent on drugs" , "Habitual drug user" , "Overly-dependent person" .)