Officer: bit of a nut, by the sound of it? (7)
I believe the answer is:
colonel
'officer' is the definition.
(I know that colonel is a type of commissioned military officer)
'bit of a nut by the sound of it?' is the wordplay.
'bit of a nut' becomes 'kernel' (edible part of a nut).
'by the sound of it?' shows a homophone (sound like).
'kernel' sounds like 'COLONEL'.
(Other definitions for colonel that I've seen before include "Senior army officer" , "'No cello for this officer, strangely (7)'" , "soldier" , "Army officer ranking below a brigadier" , "Military rank" .)