I sign under 500 books (7)
I believe the answer is:
diaries
'books' is the definition.
(diary is a kind of book)
'i sign under 500' is the wordplay.
'sign' becomes 'aries' (star sign).
'under' means one lot of letters go next to another.
's00' becomes 'd' (Roman numeral for s00).
'i'+'aries'='iaries'
'iaries' put after 'd' is 'DIARIES'.
(Other definitions for diaries that I've seen before include "Personal records of daily events" , "The Basketball _; Leonardo DiCaprio film" , "Personal journals" .)