Rent in castle as extravagant (5)
I believe the answer is:
lease
'rent' is the definition.
(I know that rent can be written as lease)
'in castle as extravagant' is the wordplay.
'in' indicates the answer is hidden within the clue.
'LEASE' can be found hidden inside 'castle as extravagant'.
(Other definitions for lease that I've seen before include "Let (property)" , "Letting contract" , "Contract for use of land or building" , "Hire out, rent" , "Term of a rental contract" .)