Demands (and gets) old book (6)
I believe the answer is:
exacts
'demands and gets' is the definition.
'old book' is the wordplay.
'old' becomes 'ex'.
'book' becomes 'acts' (I've seen this before).
'ex'+'acts'='EXACTS'
(Other definitions for exacts that I've seen before include "Insists on and obtains, say payment - sounds precise" , "Extorts" , "Compels payment of" , "Insists on or claims" , "making demands" .)