They enjoy inflicting pain, like retired detectives way back (7)
I believe the answer is:
sadists
'they enjoy inflicting pain' is the definition.
'like retired detectives way back' is the wordplay.
'like' becomes 'as'.
'retired' shows that the letters should be reversed in order.
'detectives' becomes 'dis' (Detective Inspectors, DIs).
'way' becomes 'st' (**).
'back' is a reversal indicator.
'as' back-to-front is 'sa'.
'st' back-to-front is 'ts'.
'sa'+'dis'+'ts'='SADISTS'
(Other definitions for sadists that I've seen before include "They like to hurt" , "They take pleasure form inflicting pain" , "Perverted pain-inflicters" , "deviants" , "People who enjoy inflicting pain on others" .)