Memory concerning offertory (12)
I believe the answer is:
recollection
'memory' is the definition.
(recollection is a kind of memory)
'concerning offertory' is the wordplay.
'concerning' becomes 're'.
'offertory' becomes 'collection' (I've seen this before).
're'+'collection'='RECOLLECTION'
(Other definitions for recollection that I've seen before include "Recall" , "Memory" , "Remembrance" , "mental image" .)