In favour of hiding old books at once (6)
I believe the answer is:
pronto
'at once' is the definition.
(thesaurus)
'in favour of hiding old books' is the wordplay.
'in favour of' becomes 'pro' (I've seen this before).
'hiding' indicates putting letters inside.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'books' becomes 'nt' (New Testament).
'o'+'nt'='ont'
'pro' going around 'ont' is 'PRONTO'.
(Other definitions for pronto that I've seen before include "Right now, immediately" , "Immediately, amigo" , "Proton (anag.)" , "Quickly (sl.)" , "Quick as you can, amigo" .)