Fruit – a variety of melon (5)
I believe the answer is:
lemon
'fruit' is the definition.
(lemon is a kind of fruit)
'a variety of melon' is the wordplay.
'a variety of' indicates anagramming the letters.
'melon' anagrammed gives 'LEMON'.
(Other definitions for lemon that I've seen before include "Washout" , "Fruit - as an answer it's unsatisfactory" , "definitely not a peach" , "Feeble thing (colloq.); fruit tree" , "Fruit (which is proverbially "the answer")" .)