My pen's written about former monarch, an inscrutable figure (7)
I believe the answer is:
mystery
'an inscrutable figure' 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?
'my pen's written about former monarch' is the wordplay.
'pen' becomes 'sty' (both can mean an animal enclosure).
'written about' is an insertion indicator.
'former monarch' becomes 'ER' (Elizabeth Regina, ie Queen Elizabeth).
'sty' enclosing 'er' is 'stery'.
'my'+'stery'='MYSTERY'
Can you help me to learn more?
(Other definitions for mystery that I've seen before include "Puzzling crime novel" , "Unexplained phenomenon" , "Something that passes understanding" , "Religious truth beyond human understanding" , "Secret doctrine" .)