Rare books acquired by first-class religious scholar
I believe the answer is:
rabbi
'religious scholar' is the definition.
(I've seen this before)
'rare books acquired by first-class' is the wordplay.
'rare' becomes 'r' (listed by Oxford Dictionary of Abbreviations).
'books' becomes 'bb' (two abbreviations for book).
'acquired by' is an insertion indicator.
'first class' becomes 'ai' (A1 - top quality).
'bb' placed within 'ai' is 'abbi'.
'r'+'abbi'='RABBI'
(Other definitions for rabbi that I've seen before include "Legal expert" , "Jewish law expounder" , "Religious leader of Jewish congregation" , "Sacks, for example" , "legal scholar" .)