Country finally rejecting any possible reason for complaint (4)
I believe the answer is:
germ
'reason for complaint' is the definition.
The definition and answer can be both related to thinking as well as being singular nouns.
Maybe they are linked in a way I don't understand?
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for germ that I've seen before include "Infecting microorganism" , "Micro-organism causing illness" , "Cause of illness" , "Infectious microorganism" , "Microorganism causing disease" .)