Bill turned in a murderer (4)
I believe the answer is:
cain
'a murderer' is the definition.
(Cain who killed Abel)
'bill turned in' is the wordplay.
'bill' becomes 'ac'.
'turned' is a reversal indicator.
'ac' reversed gives 'ca'.
'ca'+'in'='CAIN'
(Other definitions for cain that I've seen before include "Eve's firstborn" , "The first known murderer" , "First murderer reported in the Bible" , "Eldest son of Adam, Eve" , "First person born in Bible" .)