Priest in church demonstrates holy memento (5)
I believe the answer is:
relic
'holy memento' is the definition.
'relic' can be an answer for 'memento' (a relic is an antique or memento). I'm unsure of the 'holy' bit.
'priest in church' is the wordplay.
'priest' becomes 'eli' (biblical figure).
'in' indicates putting letters inside.
'church' becomes 'RC' (abbreviation for Roman Catholic).
'eli' going within 'rc' is 'RELIC'.
'demonstrates' is the link.
Can you help me to learn more?
(Other definitions for relic that I've seen before include "(Holy) remains" , "Object of reverence" , "Object of historic interest" , "Memorial object" , "Revered object" .)