Essence of report of army officer (6)
I believe the answer is:
kernel
'essence' is the definition.
(I've seen this before)
'report of army officer' is the wordplay.
'report of' indicates a 'sounds like' (homophone) clue.
'army officer' becomes 'colonel' (I've seen this before).
'colonel' sounds like 'KERNEL'.
'of' is the link.
(Other definitions for kernel that I've seen before include "Central or most important part" , "Contents of shell" , "Nucleus" , "Inner part of nut" , "bit of a nut" .)