Detective in criminal hustle (6)
I believe the answer is:
sleuth
'detective' is the definition.
(sleuth is a kind of detective)
'criminal hustle' is the wordplay.
'criminal' indicates an anagram.
'hustle' is an anagram of 'SLEUTH'.
'in' is the link.
(Other definitions for sleuth that I've seen before include "Holmes, for one" , "Hustle for a clever detective like Sherlock" , "Hustle (anag)" , "Gumshoe" , "Detective who follows a trail" .)