Librarian, ample, trained to become dancer (5,9)
I believe the answer is:
prima ballerina
'dancer' is the definition.
(prima ballerina is a kind of dancer)
'librarian ample trained' is the wordplay.
'trained' indicates anagramming the letters (to train can mean to grow something into a shape).
'librarian'+'ample'='librarianample'
'librarianample' anagrammed gives 'PRIMA BALLERINA'.
'to become' acts as a link.
(Other definitions for prima ballerina that I've seen before include "athletic star" , "Chief female dancer" .)