Least costly copy held in strong box (8)
I believe the answer is:
cheapest
'least costly' is the definition.
I can't tell whether this defines the answer.
'copy held in strong box' is the wordplay.
'copy' becomes 'ape' (ape can mean to copy or imitate).
'held in' indicates putting letters inside.
'strong box' becomes 'chest' (I've seen this before).
'ape' placed into 'chest' is 'CHEAPEST'.
Can you help me to learn more?
(Other definitions for cheapest that I've seen before include "Set peach in least expensive category" , "Most inferior" , "Most economical" , "Lowest in price" , "It's most shameful" .)