He investigates lush type, heartless criminal (6)
I believe the answer is:
sleuth
'he investigates' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'lush type heartless criminal' is the wordplay.
'heartless' means to remove the middle letters.
'criminal' is an anagram indicator.
'type' with its centre removed is 'te'.
'lush'+'te'='lushte'
'lushte' anagrammed gives 'SLEUTH'.
Can you help me to learn more?
(Other definitions for sleuth that I've seen before include "Detective; play and film" , "Detective who follows a trail" , "Investigator (informal)" , "Sherlock Holmes, for example" , "Gumshoe" .)