Pagan vow so ultraradical (10)
I believe the answer is:
idolatrous
'pagan' is the definition.
(thesaurus)
'vow so ultraradical' is the wordplay.
I cannot quite see how this works, but
'vow' could be 'ido' (I do, marriage vow) and 'ido' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for idolatrous that I've seen before include "Worshipping images" , "Inappropriately worshipful" , "Worshipping inappropriately" , "devoted to figures" , "worshipping wrongly" .)