Part of Spain, in truth (5)
I believe the answer is:
facet
'part' 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?
'spain in truth' is the wordplay.
'spain' becomes 'e' (abbreviation for Espana).
'in' means one lot of letters goes inside another.
'truth' becomes 'fact' (true facts).
'e' inserted into 'fact' is 'FACET'.
'of' acts as a link.
Can you help me to learn more?
(Other definitions for facet that I've seen before include "Look" , "Side of a cut gem" , "Small surface (of diamond)" , "Angle" , "Distinct feature" .)