Officer who sounds a bit of a nut (7)
I believe the answer is:
'officer' is the definition.
(colonel is a kind of officer)
'sounds a bit of a nut' is the wordplay.
'sounds' shows a homophone (sound like).
'a bit of a nut' becomes 'kernel' (edible part of a nut).
'kernel' is a homophone of 'COLONEL'.
'who' is the link.
(Other definitions for colonel that I've seen before include "Comissioned military officer" , "Rank (below brigadier)" , "Commander" , "Senior army officer" , "Military rank" .)