There's still one abominable snowman (4)
I believe the answer is:
yeti
'abominable snowman' is the definition.
(I've seen this before)
'there's still one' is the wordplay.
'there's' says to put letters next to each other.
'still' becomes 'yet' (yet can mean still or so far).
'one' becomes 'i' (Roman numeral).
'yet'+'i' is 'YETI'.
(Other definitions for yeti that I've seen before include "Legendary beast of the Himalayas?" , "imaginary beast?" , "fictional creature" , "that is found in Himalayan mythology" , "Mysterious Himalayan creature" .)