Most inferior copy discovered in box (8)
I believe the answer is:
cheapest
'most inferior' is the definition.
('cheap' can be similar in meaning to 'inferior')
'copy discovered in box' is the wordplay.
'copy' becomes 'ape' ('ape' can be a synonym of 'copy').
'discovered in' is an insertion indicator.
'box' becomes 'chest' (chest is a kind of box).
'ape' placed into 'chest' is 'CHEAPEST'.
(Other definitions for cheapest that I've seen before include "Lowest in price" , "Of the poorest quality" , "Most cut-price" , "Most inferior" , "Set peach in least expensive category" .)