Self-denying sailors can finally purchase books (9)
I believe the answer is:
Here is my best explanation:
'self-denying' is the definition.
'sailors can finally purchase books' is the subsidiary indication.
'sailors' becomes 'abs' (I've seen this in other clues).
'can' becomes 'tin' ('tin' can be a synonym of 'can'**).
'finally' says to take the final letters.
'books' becomes 'nt' (abbreviation for New Testament).
The final letter of 'purchase' is 'e'.
(Other definitions for abstinent that I've seen before include "Avoiding port, perhaps", "Restrained", "Forsaking alcohol", "Refusing alcohol", "Not taking drink", "Not indulging appetite for food or drink", "without booze", "Avoiding alcohol", "Refraining from drinking alcohol" and "On the wagon".)