Such a detective may charm, perhaps, by his manner (8)
I believe the answer is:
armchair
'his manner' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'such a detective may charm perhaps by' is the wordplay.
'such a detective may' becomes 'air' (I can't justify this - if you can you should give a lot more credence to this answer).
'perhaps' indicates anagramming the letters.
'by' means one lot of letters go next to another.
'charm' with letters rearranged gives 'armch'.
'air' put after 'armch' is 'ARMCHAIR'.
Can you help me to learn more?
(Other definitions for armchair that I've seen before include "Place for critic" , "Piece of furniture" , "Comfy seat" , "Seat to relax in" , "Amateur" .)