Heart, from which army officer's spoken? (6)
I believe the answer is:
kernel
'heart' is the definition.
(I know that heart can be written as kernel)
'army officer's spoken?' is the wordplay.
'army officer' becomes 'colonel ' (I've seen this before).
'spoken?' indicates a 'sounds like' (homophone) clue.
'colonel' sounds like 'KERNEL'.
'from which' is the link.
(Other definitions for kernel that I've seen before include "Inner part of a seed, the heart of the matter" , "The edible centre of a nut" , "Inner seed of nut" , "Essential part" , "Hard centre of nut" .)