Legendary mountain-dweller, one not appearing up till now (3)
I believe the answer is:
yet
'up till now' is the definition.
(I've seen this in another clue)
'legendary mountain-dweller one not' is the wordplay.
'legendary mountain-dweller' becomes 'yeti' (mythical creature).
'one' becomes 'i' (Roman numeral).
'not' is a deletion indicator.
'yeti' with 'i' removed is 'YET'.
'appearing' acts as a link.
(Other definitions for yet that I've seen before include "To this time, nevertheless" , "Up till now" , "Up to this time and continuing" , "Up to the present time" , "Still, so far" .)