Settlement is involved (8)
I believe the answer is:
compound
This could be a double definition.
'settlement' is the first 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 can't see?
'is involved' is the second definition.
(thesaurus)
Can you help me to learn more?
(Other definitions for compound that I've seen before include "Whole formed by two or more elements" , "See 10" , "fenced area in prison" , "Whole formed by separate elements" , "POW camp" .)