We hear an officer is at the heart of things (6)
I believe the answer is:
kernel
'at the heart of things' is the definition.
I can't tell whether this definition defines the answer.
'we hear an officer' is the wordplay.
'we hear' shows a homophone (sound like).
'an officer' becomes 'colonel' (colonel is a kind of officer).
'colonel' is a homophone of 'KERNEL'.
'is' is the link.
Can you help me to learn more?
(Other definitions for kernel that I've seen before include "Single cereal grain" , "Core of a nut" , "Central or most important part" , "Heart and soul" , "Nuts and bolts" .)