Military officer, it's said, in centre (6)
I believe the answer is:
kernel
'centre' is the definition.
(I've seen this before)
'military officer it's said' is the wordplay.
'military officer' becomes 'colonel' (colonel is a kind of military officer).
'it's said' indicates a 'sounds like' (homophone) clue.
'colonel' is a homophone of 'KERNEL'.
'in' is the link.
(Other definitions for kernel that I've seen before include "Inner part of a seed, the heart of the matter" , "Hard centre of nut" , "Nuts and bolts" , "Inner seed of nut" , "Essential part" .)