Graduate left lesson in castle (8)
I believe the answer is:
balmoral
'castle' is the definition.
(Balmoral Castle, royal residence)
'graduate left lesson' is the wordplay.
'graduate' becomes 'BA' (someone with a Bachelor of Arts degree).
'left' becomes 'l' (common abbreviation).
'lesson' becomes 'moral' (as in the moral of the story).
'ba'+'l'+'moral'='BALMORAL'
'in' is the link.
(Other definitions for balmoral that I've seen before include "Scottish residence of the British sovereign" , "Bonnet" , "Royal retreat in Scotland" , "Queen's address?" , "Royal holiday residence on the River Dee" .)