Soldier said to be a bit of a nut (7)
I believe the answer is:
colonel
'soldier' is the definition.
(military rank)
'said to be a bit of a nut' is the wordplay.
'said to be' shows a homophone (sound like).
'a bit of a nut' becomes 'kernel' (edible part of a nut).
'kernel' sounds like 'COLONEL'.
(Other definitions for colonel that I've seen before include "Bogey perhaps" , "Army officer ranking below a brigadier" , "No cello (anag.)" , "Army rank" , "High-ranking officer" .)