Still one monster (4)
I believe the answer is:
yeti
'monster' is the definition.
(I know that yeti is a type of legendary creature)
'still one' is the wordplay.
'still' becomes 'yet' (yet can mean still or so far).
'one' becomes 'I' (Roman numeral).
'yet'+'i'='YETI'
(Other definitions for yeti that I've seen before include "Creature perhaps" , "strange creature" , "fictional creature" , "elusive character" , "Feared creature" .)