Short boy munches a vegetable (4)
I believe the answer is:
bean
'vegetable' is the definition.
(bean is a kind of vegetable)
'short boy munches a' is the wordplay.
'short boy' becomes 'Ben' (Ben might be short for Benjamin).
'munches' is an insertion indicator (inserted letters are eaten or munched).
'ben' placed around 'a' is 'BEAN'.
(Other definitions for bean that I've seen before include "Pulse" , "popular character with kids" , "Haricot or runner?" , "Fava, kidney or mung, say" , "Kidney-shaped seed as for coffee" .)