Fellow involved in robbery is a pleasure-seeker (8)
I believe the answer is:
hedonist
'pleasure-seeker' is the definition.
(thesaurus)
'fellow involved in robbery' is the wordplay.
'fellow' becomes 'don' (I've seen this before).
'involved in' indicates putting letters inside.
'robbery' becomes 'heist' (heist is a kind of robbery).
'don' inserted inside 'heist' is 'HEDONIST'.
'is a' is the link.
(Other definitions for hedonist that I've seen before include "sybarite" , "Seeker of sensual pleasure" , "Self-indulgent type" , "Thrill-seeker" , "One who lives only for pleasure" .)