Still, one that’s never been authenticated (4)
I believe the answer is:
yeti
'that's never been authenticated' is the definition.
'still one' is the wordplay.
'still' becomes 'yet' ('yet' can be a synonym of 'still').
'one' becomes 'i' (Roman numeral).
'yet'+'i'='YETI'
(Other definitions for yeti that I've seen before include "Himalayan enigma" , "supposed mountain creature" , "Mysterious Himalayan creature" , "... and creature supposedly there" , "Your claim to have seen one wouldn't be believed" .)