Tradition has to include mass retreat (9)
I believe the answer is:
hermitage
'retreat' is the definition.
(a hermitage may be a religious retreat)
'tradition has to include mass' is the wordplay.
'tradition' becomes 'heritage' (I've seen this before).
'has' is an insertion indicator.
'to include mass' becomes 'm' (I can't justify this - if you can you should believe this answer much more).
'heritage' enclosing 'm' is 'HERMITAGE'.
Can you help me to learn more?
(Other definitions for hermitage that I've seen before include "Recluse's abode" , "St Petersburg museum - Rhone wine" , "Home of a recluse" , "art museum" , "Place of retreat" .)