Wanting part from autopsy, choose nut (6)

I believe the answer is:

psycho

I'm a little stuck... Click here to teach me more about this clue!

'wanting part' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.

'from autopsy choose nut' is the wordplay.
'from' indicates a hidden word.
'PSYCHO' can be found hidden inside 'autopsy choose nut'.

Can you help me to learn more?

(Other definitions for psycho that I've seen before include "Bates Motel horror film" , "Loony" , "Hichcock film" , "Hitchcock film with Anthony Perkins" , "Shy cop (anag) - 1960 movie" .)

I've seen this clue in The Independent.

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