Crime's unravelled by Arnott oddly finding criminal (9)
I believe the answer is:
miscreant
'criminal' is the definition.
(I've seen this before)
'crime's unravelled by arnott oddly' is the wordplay.
'unravelled' is an anagram indicator.
'by' is a charade indicator (letters next to each other).
'oddly' indicates alternate letters.
The alternate letters of 'arnott' are 'ant'.
'crimes' with letters rearranged gives 'miscre'.
'miscre'+'ant'='MISCREANT'
'finding' is the link.
(Other definitions for miscreant that I've seen before include "villain" , "culprit" , "Naughty boy!" , "scoundrel" , "behaving unlawfully" .)