Snack food that's gone stale? (8)
I believe the answer is:
chestnut
This clue is a double definition.
'snack food' is the first definition.
'that's gone stale?' is the second definition.
(a chestnut is a hackneyed idea or phrase)
(Other definitions for chestnut that I've seen before include "''Underneath the spreading . . . . . . . . tree''" , "Tree (horse or sweet)" , "Laughably predictable" , "comedian's overused line?" , "Tree; old joke (colloq.)" .)