Demands (and gets) old book (6)
I believe the answer is:
'demands and gets' is the definition.
'old book' is the wordplay.
'old' becomes 'ex'.
'book' becomes 'acts' (I've seen this before).
(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" .)