The CO sounds a bit of a nut (6)
I believe the answer is:
kernel
'bit of a nut' is the definition.
(edible part of a nut)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for kernel that I've seen before include "Stone" , "Nuts and bolts" , "Contents of shell" , "Edible seed of nut" , "Nut's centre" .)