Book entering system, marked (4)
I believe the answer is:
emma
'book' is the definition.
(Jane Austen novel)
'entering system marked' is the wordplay.
'entering' indicates the answer is hidden within the clue.
'EMMA' is hidden within 'system marked'.
(Other definitions for emma that I've seen before include "Miss Woodhouse?" , "young woman in novel" , "Jane Austen work - Ms Bunton, former Baby Spice" , "Ack -, a term for a.m." , "Jane Austen novel and Nelson's lover" .)