Miss engrossed by court secret (6)
I believe the answer is:
closet
'secret' is the definition.
('closet' can be a synonym of 'secret')
'miss engrossed by court' is the wordplay.
'miss' becomes 'lose' (I've seen this in other clues**).
'engrossed by' means one lot of letters goes inside another (I've seen this in other clues).
'court' becomes 'ct'.
'lose' put within 'ct' is 'CLOSET'.
(Other definitions for closet that I've seen before include "Furtive" , "Private room; cupboard" , "US press for clothing" , "Study" , "Cupboard (to come out of)" .)