Where one may study royal fashion (9)
I believe the answer is:
princeton
'where one may study' is the definition.
(US university)
'royal fashion' is the wordplay.
'royal' becomes 'prince' (prince is a kind of royal).
'fashion' becomes 'ton' (I've seen this in other clues).
'prince'+'ton'='PRINCETON'
(Other definitions for princeton that I've seen before include "Early US university" , "American university" , "College" , "US ivy league university" , "Member of the 11 in New Jersey" .)