Service, say, involving archdeacon and offspring (8)
I believe the answer is:
evensong
'service' is the definition.
(type of church service)
'say involving archdeacon and offspring' is the wordplay.
'say' becomes 'eg' (both can mean 'for example').
'involving' means one lot of letters goes inside another.
'archdeacon' becomes 'ven' (abbreviation for The Venerable).
'and' is a charade indicator (letters next to each other).
'offspring' becomes 'son' (male offspring).
'ven'+'son'='venson'
'eg' placed around 'venson' is 'EVENSONG'.
(Other definitions for evensong that I've seen before include "Service of evening prayer in Church of England" , "... Prayers" , "Late afternoon church service" , "Anglican service" , "Religious service" .)