Inscrutability of the writer’s pen, capturing queen (7)
I believe the answer is:
mystery
'inscrutability' is the definition.
(thesaurus)
'writer's pen capturing queen' is the wordplay.
'writer's' becomes 'my' (belonging to the person who set the crossword).
'pen' becomes 'sty' (both can mean an animal enclosure).
'capturing' means one lot of letters goes inside another.
'queen' becomes 'ER' (abbreviation for Elizabeth Regina).
'sty' placed around 'er' is 'stery'.
'my'+'stery'='MYSTERY'
'of the' acts as a link.
(Other definitions for mystery that I've seen before include "Religious rite - something inexplicable" , "Mediaeval trade" , "Something hard to explain" , "Puzzle" , "Secret doctrine" .)