Revolution helping to protect religious books (8)
I believe the answer is:
rotation
'revolution' is the definition.
(synonyms)
'helping to protect religious books' is the wordplay.
'helping' becomes 'ration' (a ration is a serving or helping).
'to protect' means one lot of letters goes inside another.
'religious books' becomes 'ot' (Old Testament).
'ration' enclosing 'ot' is 'ROTATION'.
(Other definitions for rotation that I've seen before include "Turn of a wheel" , "Spinning motion or cyclic changes as of crops" , "Revolutionary movement" , "Action of turning" , "Turning movement around an axis" .)