Day book's last prayer (4)
I believe the answer is:
monk
'prayer' is the definition.
(someone who prays)
'day book's last' is the wordplay.
'day' becomes 'mon' (abbreviation for Monday).
'last' says to take the final letters.
The final letter of 'book' is 'k'.
'mon'+'k'='MONK'
(Other definitions for monk that I've seen before include "Trappist, for example" , "Tonsured person" , "One in a religious order" , "prayer?" , "One vowed" .)