Melon that's hard held up by weedy criminal (8)
I believe the answer is:
honeydew
'melon' is the definition.
(honeydew melon is a kind of melon)
'hard held up by weedy criminal' is the wordplay.
'hard' becomes 'hon' (I can't explain this - if you can you should give a lot more credence to this answer).
'held up by' says to put letters next to each other (in a down clue, some letters hold others up).
'criminal' indicates anagramming the letters.
'weedy' with letters rearranged gives 'eydew'.
'hon'+'eydew'='HONEYDEW'
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for honeydew that I've seen before include "Sweet substance produced by aphids" , "Type of melon" , "Food supplied and eaten by 20s (and Kubla Khan?)" , "Melon - Owen Hyde (anag)" , "Melon variety" .)