Old priest in Roman Catholic memorial (5)
I believe the answer is:
relic
'memorial' is the definition.
(object kept to remember a holy person)
'old priest in roman catholic' is the wordplay.
'old priest' becomes 'eli' (Biblical character).
'in' is an insertion indicator.
'roman catholic' becomes 'rc' (abbreviation for Roman Catholic).
'eli' put inside 'rc' is 'RELIC'.
(Other definitions for relic that I've seen before include "Personal saintly memorial" , "Something from saint" , "Holy remnant" , "Corporal remains" , "Outdated survival" .)