He has a fiendish way of working in a haunt of vice (5)
I believe the answer is:
demon
'he has a fiendish way of working in a haunt of vice' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'he has a fiendish way of working in' is the wordplay.
'he' becomes 'Em'.
'has' says to put letters next to each other.
'a fiendish way of' becomes 'd' (I can't explain this - if you can you should believe this answer much more).
'working in' becomes 'on' (I've seen this in other clues).
'em' put after 'd' is 'dem'.
'dem'+'on'='DEMON'
Can you help me to learn more?
(Other definitions for demon that I've seen before include "See 3" , "ace" , "Evil spirit or cruel person" , "Imp" , "Fallen angel" .)