Priest brought in Roman Catholic memento (5)
I believe the answer is:
relic
'memento' is the definition.
(a relic is an antique or memento)
'priest brought in roman catholic' is the wordplay.
'priest' becomes 'eli' (biblical figure).
'brought in' means one lot of letters goes inside another.
'roman catholic' becomes 'RC' (abbreviation for Roman Catholic).
'eli' put inside 'rc' is 'RELIC'.
(Other definitions for relic that I've seen before include "Memorial of saint" , "Ancient and venerated object" , "Memorial object" , "Fossil" , "Trace" .)