One against religious image (4)
I believe the answer is:
icon
'religious image' is the definition.
(I've seen this before)
'one against' is the wordplay.
'one' becomes 'ic' (I can't explain this - if you can you should give a lot more credence to this answer).
'against' becomes 'on' (I've seen this in other clues).
'ic'+'on'='ICON'
Can you help me to learn more?
(Other definitions for icon that I've seen before include "Someone uncritically admired" , "Painting or person regarded as sacred" , "Image or statue, or symbol on computer screen" , "Russian religious image" , "Very important person" .)