Fellow into crime, pleasure-seeker (8)
I believe the answer is:
hedonist
'pleasure-seeker' is the definition.
(thesaurus)
'fellow into crime' is the wordplay.
'fellow' becomes 'don' (I've seen this before).
'into' indicates putting letters inside.
'crime' becomes 'heist' (heist is a kind of crime**).
'don' placed within 'heist' is 'HEDONIST'.
(Other definitions for hedonist that I've seen before include "Self-indulgent type" , "playboy" , "one enjoying himself" , "A seeker after luxury" , "For whom pleasure is the chief god" .)