Evil is returning with nefarious leader (3)
I believe the answer is:
sin
'evil' is the definition.
(thesaurus)
'is returning with nefarious leader' is the wordplay.
'returning' says the letters should be written in reverse.
'with' means one lot of letters go next to another.
'leader' indicates taking the first letters.
The first letter of 'nefarious' is 'n'.
'is' in reverse letter order is 'si'.
'si'+'n'='SIN'
(Other definitions for sin that I've seen before include "Mortal offence" , "Mortal error" , "Immorality" , "Wickedness" , "Behave badly" .)