Melon's different fruit (5)
I believe the answer is:
lemon
'fruit' is the definition.
(I know that lemon is a fruit)
'melon's different' is the wordplay.
'different' indicates an anagram.
'melon' anagrammed gives 'LEMON'.
(Other definitions for lemon that I've seen before include "Fruit; sort of grass, sole" , "Sharp-tasting fruit" , "A fruit; a worthless item" , "Citrus fruit (is it the answer?)" , "Acidic fruit" .)