Heard officer is a nut (6)
I believe the answer is:
kernel
'nut' is the definition.
(I've seen this in another clue)
'heard officer' is the wordplay.
'heard' shows a homophone (sound like).
'officer' becomes 'colonel' (colonel is a kind of officer).
'colonel' is a homophone of 'KERNEL'.
'is a' is the link.
(Other definitions for kernel that I've seen before include "Nucleus" , "Inner part of nut" , "Edible seed of nut" , "Hard centre of nut" , "Nut's centre" .)