Revise holy book, deleting additional words (5)
I believe the answer is:
alter
'revise' is the definition.
(I've seen this in another clue)
'holy book deleting additional words' is the wordplay.
'holy book' becomes 'psalter' (a copy of the Book of Psalms).
'deleting' suggests deleting specific letters.
'additional words' becomes 'PS' (as in a PS in a letter).
'psalter' with 'ps' taken away is 'ALTER'.
(Other definitions for alter that I've seen before include "Adapt" , "and 10 Across: Second self" , "reform" , "convert" , "Bout of extravagant spending" .)