Maybe one pardoning sins to which man’s been prone? (8)
I believe the answer is:
Here is my best explanation:
'maybe one pardoning sins' is the definition.
'man's been prone?' is the wordplay.
'man' becomes 'chap' (I've seen this in other clues).
'been prone?' becomes 'lain'.
'to which' acts as a link.
(Other definitions for chaplain that I've seen before include "cleric", "Member of the clergy, in 24 ac say", "Minister", "Member of the clergy attached to an institution", "Regimental clergyman", "Clergyman attached to military unit", "An army clergyman", "Christian clergyman attached to an institution, military body etc", "Clergyman attached to an institution, such as the army" and "Religious minister assigned to an organisation".)