Book already commencing, the story starts (4)
I believe the answer is:
acts
'book' is the definition.
(I've seen this before)
'already commencing the story starts' is the wordplay.
'starts' says to take the initial letters.
The first letters of 'already commencing the story' is 'ACTS'.
(Other definitions for acts that I've seen before include "religious work" , "No head for details in book" , "Cast in exploits" , "Decrees" , "Follower of John" .)