Book expert, one under one zodiacal sign not initially following another (9)
I believe the answer is:
librarian
'book expert' is the definition.
'one under one zodiacal sign not initially following another' is the wordplay.
'one under' says to put letters next to each other.
'one zodiacal sign' becomes 'libra' (I've seen this before).
'not initially' becomes 'ian' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'following' is a charade indicator (letters next to each other).
'another' becomes 'r' (I've seen this in other clues).
'ian' after 'r' is 'rian'.
'libra'+'rian' is 'LIBRARIAN'.
(Other definitions for librarian that I've seen before include "Bibliothecary" , "employee in Reading office?" , "Bookkeeper" , "Person working with books" , "One dealing with books" .)