Sign in book store missing line (5)
I believe the answer is:
libra
'sign' is the definition.
(sign of the zodiac)
'book store missing line' is the wordplay.
'book store' becomes 'library' (a store of books).
'missing' indicates named letters should be taken away.
'line' becomes 'ry' (abbreviation for railway).
'library' with 'ry' taken away is 'LIBRA'.
'in' is the link.
(Other definitions for libra that I've seen before include "Old pound" , "Zodiacal sign" , "House" , "Person born under Scales sign" , "The Scales in astrology" .)