Still one sees 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 mean still or so far).
'one' becomes 'I' (Roman numeral).
'yet'+'i'='YETI'
'sees' acts as a link.
(Other definitions for yeti that I've seen before include "that you might find in mountains" , "something abominable" , "highland inhabitant" , "Highly elusive monster" , "whose existence is unconfirmed" .)