Hustler almost confused a private detective (6)
I believe the answer is:
sleuth
'a private detective' is the definition.
'hustler almost confused' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'confused' indicates an anagram.
'hustler' with its final letter taken off is 'hustle'.
'hustle' anagrammed gives 'SLEUTH'.
(Other definitions for sleuth that I've seen before include "Hustle like detective (6)" , "Track" , "Detective who follows a trail" , "A detective like Sherlock Holmes" , "Hustle (anag)" .)