Priest's work: lecture on sin, Monday off sinning (7)
I believe the answer is:
service
'priest's work' is the definition.
'lecture on sin monday off sinning' is the wordplay.
'lecture on sin' becomes 'sermon'.
'monday' becomes 'mon' ('Mon' can be a synonym of 'monday').
'off' suggests deleting specific letters.
'sinning' becomes 'vice'.
'sermon' with 'mon' taken away is 'ser'.
'ser'+'vice'='SERVICE'
(Other definitions for service that I've seen before include "tree" , "Church ceremony" , "Set of dishes" , "funeral, say" , "mass, say" .)