Say Marvin eats sweet vegetable (6)
I believe the answer is:
legume
'vegetable' is the definition.
(I know that legume is a type of vegetable)
'say marvin eats sweet' is the wordplay.
'say marvin' becomes 'lee' (I can't justify this - if you can you should give a lot more credence to this answer).
'eats' is an insertion indicator.
'sweet' becomes 'gum' (gum is a kind of sweet).
'lee' enclosing 'gum' is 'LEGUME'.
Can you help me to learn more?
(Other definitions for legume that I've seen before include "Plant that bears its seeds in a pod" , "Veg as pea, bean" , "A vegetable for e.g. mule" , "Vegetable of pea family" , "Pea or bean pod" .)