Guys eating most of blue fruit (5)
I believe the answer is:
melon
'fruit' is the definition.
(melon is a kind of fruit)
'guys eating most of blue' is the wordplay.
'guys' becomes 'men'.
'eating' means one lot of letters goes inside another.
'most of' means to remove the last letter.
'blue' becomes 'low' (I've seen this in other clues).
'low' with its final letter taken off is 'lo'.
'men' enclosing 'lo' is 'MELON'.
(Other definitions for melon that I've seen before include "Type of pepo" , "Cantaloupe fruit" , "Large round fruit" , "Sweet juicy gourd" , "Edible gourd" .)