Bookkeeper's sign -- managed to get hold of one (9)
I believe the answer is:
'bookkeeper's' is the definition.
'sign managed to get hold of one' is the wordplay.
'sign' becomes 'libra' (sign of the zodiac**).
'managed' becomes 'ran' (I've seen this before).
'to get hold of' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'ran' enclosing 'i' is 'rian'.
(Other definitions for librarian that I've seen before include "Philip Larkin was one at Hull University" , "Person in charge of books etc" , "Custodian of a collection of books" , "job!" , "Bookkeeper" .)