The most common and most likely answer to this crossword clue is the 8 letter word HONEYDEW
I've seen this crossword clue in The Daily Telegraph
H
O
N
E
Y
D
E
W
Buddy explains!

The straight part
We know that "Melon" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case melon = honeydew.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does HONEYDEW mean?
HONEYDEW noun- A sweet sticky substance deposited on leaves by insects.
- A sweet sticky substance produced by the leaves of some plants.
- A melon with sweet green flesh, with a smooth greenish-white exterior, of cultivar group Cucumis melo Inodorus group.
- A light bluish-green colour, like that of a honeydew melon.
- A fine sort of tobacco moistened with molasses.
- Of a light bluish green colour, like that of a honeydew melon.