Discussed senior officer's germ (6)
I believe the answer is:
kernel
'germ' is the definition.
The definition and answer can be both related to thinking as well as being singular nouns.
Perhaps you can see an association between them that I don't see?
'discussed senior officer's' is the wordplay.
'discussed' indicates a 'sounds like' (homophone) clue.
'senior officer' becomes 'colonel'.
'colonel' sounds like 'KERNEL'.
Can you help me to learn more?
(Other definitions for kernel that I've seen before include "Core of a nut" , "Nuts and bolts" , "soft heart" , "Essential part" , "Inner part of a seed or grain" .)