Nut — as in nibble? (6)
I believe the answer is:
cashew
'nut' is the definition.
(I know that cashew is a type of edible nut)
'as in nibble?' is the wordplay.
'in' indicates putting letters inside.
'nibble?' becomes 'chew' (I can't justify this - if you can you should believe this answer much more).
'as' inserted into 'chew' is 'CASHEW'.
Can you help me to learn more?
(Other definitions for cashew that I've seen before include "Chew as you would this nut" , "Acajou" , "Kidney-shaped nut of tropical tree" , "nut-tree" , "Kidney shaped nut" .)