Toddler munches a small piece of brown bread (5)
I believe the answer is:
toast
'piece of brown bread' is the definition.
(piece of bread that has been browned)
'toddler munches a small' is the wordplay.
'toddler' becomes 'tot' (synonyms).
'munches' means one lot of letters goes inside another (inserted letters are eaten or munched).
'small' becomes 's' (abbreviation - e.g. clothes size).
'a'+'s'='as'
'tot' enclosing 'as' is 'TOAST'.
(Other definitions for toast that I've seen before include "Warm, it may be eaten" , "Cheers" , "It's found under marmalade and butter" , "'----- of the Town': one receiving the acclaim of one's peers" , "Formally drink a health" .)