Boy eating a vegetable (4)
I believe the answer is:
bean
'vegetable' is the definition.
(bean is a kind of vegetable)
'boy eating a' is the wordplay.
'boy' becomes 'ben' (boy's name).
'eating' is an insertion indicator.
'ben' placed around 'a' is 'BEAN'.
(Other definitions for bean that I've seen before include "Nut" , "popular character with kids" , "See 12 Down" , "Edible seed" , "Plant with seed in pod" .)