He has his own way of working in crime (5)
I believe the answer is:
simon
'he has his own' is the definition.
The definition suggests a singular noun which matches the answer.
'way of working in crime' is the wordplay.
'way of working' becomes 'mo' (abbreviation for modus operandi).
'in' means one lot of letters goes inside another.
'crime' becomes 'sin'.
'mo' put within 'sin' is 'SIMON'.
Can you help me to learn more?
(Other definitions for simon that I've seen before include "- Bol'var, S. American liberator" , "He's simple in rhyme" , "One of Christ's apostles" , "Simple chap" , "Bolivar, de Montfort, Callow or Cowell?" .)