What he studies is wrong (13)
I believe the answer is:
criminologist
'what he studies is wrong' is the definition.
The definition suggests a singular noun which matches the answer.
'he studies is wrong' is the wordplay.
I cannot quite understand how this works, but
'is' is within the answer.
'wrong' could be 'no' ('no' can mean what just someone said is wrong) and 'no' is present in the answer.
'he' could be 'milo' and 'milo' is present in the leftover letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for criminologist that I've seen before include "Expert in breaking the law?" , "One studying felons" , "One studying law-breakers" , "One who studies felons" , "Low-life expert" .)