Legendary creature, still first in index (4)
I believe the answer is:
yeti
'legendary creature' is the definition.
(yeti is a kind of legendary creature)
'still first in index' is the wordplay.
'still' becomes 'yet' ('yet' can be a synonym of 'still').
'first in' says to take the initial letters.
The first letter of 'index' is 'i'.
'yet'+'i'='YETI'
(Other definitions for yeti that I've seen before include "Mythical Tibetan beast" , "mythical creature" , "Hairy recluse" , "The Abominable Snowman in Tibetan" , "Tibetan creature" .)