Religious images one studies (5)
I believe the answer is:
icons
'religious images' is the definition.
(I've seen this before)
'one studies' is the wordplay.
'one' becomes 'i' (Roman numeral).
'studies' becomes 'cons' (to con is to study or revise).
'i'+'cons'='ICONS'
(Other definitions for icons that I've seen before include "Religious paintings or computer symbols" , "Celebrities" , "Holy paintings" , "Devotional paintings" , "Religious or computer images" .)