Keen follower has to change round books (6)
I believe the answer is:
votary
'keen follower' is the definition.
'to change round books' is the wordplay.
'to change' becomes 'vary' ('vary' can be a synonym of 'change').
'round' is an insertion indicator.
'books' becomes 'ot' (Old Testament).
'vary' going around 'ot' is 'VOTARY'.
'has' is the link.
(Other definitions for votary that I've seen before include "Devoted adherent of a religion" , "Devoted follower" , "Monk or nun who has taken vows" , "One by promise bound" , "One dedicated to cause, religion" .)