Historical object Roman Catholic priest locked inside (5)
I believe the answer is:
relic
'historical object' is the definition.
(I've seen this before)
'roman catholic priest locked inside' is the wordplay.
'roman catholic' becomes 'rc' (abbreviation for Roman Catholic).
'priest' becomes 'eli' (biblical figure).
'locked inside' is an insertion indicator.
'rc' placed around 'eli' is 'RELIC'.
(Other definitions for relic that I've seen before include "Surviving artefact" , "Survivor from the past" , "Holy fragment as of saint" , "Ancient artefact" , "Outdated survival" .)