One enters grand residence in part of Spain (7)
I believe the answer is:
castile
'part of spain' is the definition.
(I know that Castile is part of Spain)
'one enters grand residence' is the wordplay.
'one' becomes 'i' (Roman numeral).
'enters' indicates putting letters inside.
'grand residence' becomes 'castle' (eg a royal residence).
'i' put into 'castle' is 'CASTILE'.
'in' is the link.
(Other definitions for castile that I've seen before include "Old Spanish kingdom" , "Elastic region in central Spain" , "former Spanish kingdom" , "Elastic (anag) -- region of Spain" .)