Absence of archdeacon on a religious devotion (6)
I believe the answer is:
novena
'religious devotion' is the definition.
(form of worship over nine days)
'absence of archdeacon on a' is the wordplay.
'absence of' becomes 'no'.
'archdeacon' becomes 'ven' (abbreviation for The Venerable).
'on' means one lot of letters go next to another.
'no'+'ven'+'a'='NOVENA'
(Other definitions for novena that I've seen before include "Nine-day ritual" , "RC devotion over nine days" , "Catholic nine-day devotion" , "RC prayers over 9 days" , "Nine consecutive days of prayer or worship" .)