Book old hat, a model admitted (5)
I believe the answer is:
title
'book' is the definition.
(I've seen this before)
'old hat a model admitted' is the wordplay.
'old hat' becomes 'tile' (tile is an old-fashioned term for hat).
'a model' becomes 'T' (Ford Model T car).
'admitted' means one lot of letters goes inside another (inserted letters get admitted).
'tile' placed around 't' is 'TITLE'.
(Other definitions for title that I've seen before include "Book's name" , "big role" , "The name of a work of art or book" , "Name, honorific" , "One published" .)