Still one mythical creature? (4)
I believe the answer is:
yeti
'mythical creature?' is the definition.
(yeti is a kind of mythical creature)
'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 "Mysterious Himalayan creature" , "An abominable animal!" , "something abominable" , "Himalayan enigma" , "Mythical Himalayan beast" .)