Just imaginary, ultimately, mythological being (5)
I believe the answer is:
fairy
'mythological being' is the definition.
'just imaginary ultimately' is the wordplay.
'just' becomes 'fair' (just can mean fair in the sense of showing justice).
'ultimately' suggests the final letters.
The final letter of 'imaginary' is 'y'.
'fair'+'y'='FAIRY'
(Other definitions for fairy that I've seen before include "Creature of folklore" , "- Cake, - godmother" , "Mythical being" , "Sprite with a toadstool for a home" , "Magical being" .)