Historical object shown by RC priest inside (5)
I believe the answer is:
relic
'historical object' is the definition.
(I've seen this before)
'rc priest inside' is the wordplay.
'priest' becomes 'eli' (I've seen this before).
'inside' means one lot of letters goes inside another.
'rc' enclosing 'eli' is 'RELIC'.
'shown by' is the link.
(Other definitions for relic that I've seen before include "Part of a holy person's body or possession" , "Saint's body part?" , "Souvenir; survival" , "Holy remnant" , "Surviving ancient custom" .)