Report of senior officer — a bit of a nut! (6)
I believe the answer is:
kernel
'a bit of a nut' is the definition.
(edible part of a nut)
'report of senior officer' is the wordplay.
'report of' shows a homophone (sound like).
'senior officer' becomes 'colonel'.
'colonel' sounds like 'KERNEL'.
(Other definitions for kernel that I've seen before include "Single cereal grain" , "soft heart" , "Contents of shell" , "Seed in hard shell" , "Nuts and bolts" .)