Black just capturing a learner's castle (8)
I believe the answer is:
balmoral
'castle' is the definition.
(Balmoral Castle, royal residence)
'black just capturing a learner's' is the wordplay.
'black' becomes 'b' (as in pencil classifications).
'just' becomes 'moral' (moral can mean just or fair).
'capturing' indicates putting letters inside.
'learner' becomes 'l' (as in a learner driver with L-plates on their car).
'b'+'moral'='bmoral'
'a'+'l'='al'
'bmoral' going around 'al' is 'BALMORAL'.
(Other definitions for balmoral that I've seen before include "Queen's address?" , "Queen's Scottish home" , "Royal family's holiday residence in Scotland" , "Laced boot" , "Scottish royal home" .)