A book accepted by European athlete (7)
I believe the answer is:
acrobat
'athlete' is the definition.
(acrobat is a kind of athlete)
'a book accepted by european' is the wordplay.
'book' becomes 'b' (abbreviation for book).
'accepted by' indicates putting letters inside.
'european' becomes 'croat' (person from Croatia).
'b' inserted inside 'croat' is 'crobat'.
'a'+'crobat'='ACROBAT'
(Other definitions for acrobat that I've seen before include "Athletic type" , "circus artiste" , "Gymnastic performer" , "Athletic Circus Performer" , "Circus gymnast" .)