Old books brought up in addition (3)
I believe the answer is:
too
'in addition' is the definition.
(thesaurus)
'old books brought up' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'books' becomes 'ot' (Old Testament).
'brought up' shows that the letters should be reversed in order (down clues only).
'o'+'ot'='oot'
'oot' in reverse letter order is 'TOO'.
(Other definitions for too that I've seen before include "What a pity!" , "Additionally" , "Also; overly" , "Moreover, in addition" , "Excessively" .)