Driving force of fellows in books — protected by mother (8)
I believe the answer is:
momentum
'driving force' is the definition.
(impetus or force of a moving object)
'fellows in books protected by mother' is the wordplay.
'fellows' becomes 'men' (fellow can mean a man).
'in' indicates putting letters inside.
'books' becomes 'OT' (Old Testament).
'protected by' indicates putting letters inside.
'mother' becomes 'mum' (informal term for mother).
'men' inserted within 'ot' is 'oment'.
'oment' put inside 'mum' is 'MOMENTUM'.
'of' acts as a link.
(Other definitions for momentum that I've seen before include "Impetus from motion" , "Impelling force" , "Corbyn supporters" , "Drive" , "Forward impetus" .)