The CO, we hear, is a bit of a nut (6)

I believe the answer is:

kernel

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'is a bit of a nut' is the definition.
(I've seen this before)

I don't understand the rest of the clue.

(Other definitions for kernel that I've seen before include "Inner part of a seed, the heart of the matter" , "Essential part" , "Nuts and bolts" , "Core of a nut" , "Inner part of a seed or grain" .)

I've seen this clue in the Evening Standard.

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