Upper-class scholar primarily associated with bird book (5)
I believe the answer is:
titus
'book' is the definition.
(the Epistle of Paul to Titus, a book of the Bible)
'upper-class scholar primarily associated with bird' is the wordplay.
'upper class' becomes 'u' (as in U- and non-U speech).
'primarily' suggests taking the first letters.
'associated with' says to put letters next to each other.
'bird' becomes 'tit' (tit is a kind of bird).
The first letter of 'scholar' is 's'.
'u'+'s'='us'
'us' put after 'tit' is 'TITUS'.
(Other definitions for titus that I've seen before include "Only part of Shakespeare play" , "Groan, possibly" , "identifying Philemon's neighbour" , "Roman emperor from AD79" , "& 20 Mervyn Peake character" .)