Munch, eating a small nut (6)
I believe the answer is:
cashew
'nut' is the definition.
(cashew is a kind of nut)
'munch eating a small' is the wordplay.
'munch' becomes 'chew' (I've seen this before).
'eating' means one lot of letters goes inside another.
'small' becomes 's' (abbreviation - e.g. clothes size).
'a'+'s'='as'
'chew' placed around 'as' is 'CASHEW'.
(Other definitions for cashew that I've seen before include "Kidney-shaped nut of tropical tree" , "Kidney shaped nut" , "Acajou" , "nut-tree" , "Common type of nut" .)