Nothing called the ultimate fruit (6)
I believe the answer is:
orange
'fruit' is the definition.
(orange is a kind of fruit)
'nothing called the ultimate' is the wordplay.
'nothing' becomes 'o' (looks like zero - 0).
'called' becomes 'rang' ('ring' can be a synonym of 'call').
'ultimate' indicates one should take the final letters.
The final letter of 'the' is 'e'.
'o'+'rang'+'e'='ORANGE'
(Other definitions for orange that I've seen before include "Colour between red and yellow" , "Colour of an onager?" , "Go earn the fruit" , "A rainbow colour" , "Belisha beacon colour" .)