Sets about leaving OT book in assembly (8)

I believe the answer is:

ecclesia

'sets about leaving ot' is the definition.
I can't judge whether this definition defines the answer.

'book in assembly' is the wordplay.
'book' becomes 'eccles' (short for Ecclesiastes, book of the Old Testament).
'in assembly' becomes 'ia' (this might be a standard abbreviation I've not previously seen).
'eccles'+'ia'='ECCLESIA'

(Another definition for ecclesia that I've seen is " Church congregation".)

I've seen this clue in The Times.

Want a hint initially instead of a full solution? Install my app