Religious adherent probes contrary, loud nihilist (5)
I believe the answer is:
hindu
'religious adherent' is the definition.
(I've seen this before)
'probes contrary loud nihilist' is the wordplay.
'contrary' shows that the letters should be reversed in order.
'probes' says the answer is hidden in the clue (I've seen 'probing' mean this (the answer probes or pokes into the clue)).
'udnih' is hidden within 'loud nihilist'.
'udnih' in reverse letter order is 'HINDU'.
(Other definitions for hindu that I've seen before include "Believer in South Asian religion" , "Man of religion" , "Believer in an Indian religion" , "Member of great Indian religion with belief in reincarnation" , "Adherent of Indian religion" .)