Musician’s assistant’s gripping novel (4-6)
I believe the answer is:
page-turner
I believe this clue is a double definition.
'musician's assistant's' is the first definition.
(someone turning pages for a musician)
'gripping novel' is the second definition.
(turn for an exciting novel)
(Other definitions for page-turner that I've seen before include "Book one can't put down" , "'Unputdownable' book" , "exciting book" , "Gripping book" .)