Legendary creature, until now, first in index (4)
I believe the answer is:
yeti
'legendary creature' is the definition.
(yeti is a kind of legendary creature)
'until now first in index' is the wordplay.
'until now' becomes 'yet' (I've seen this before).
'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 "creature awaiting discovery?" , "that is found in Himalayan mythology" , "An abominable animal!" , "Mythical humanoid" , "Legendary beast of the Himalayas?" .)