Box containing old film (7)
I believe the answer is:
cartoon
'film' is the definition.
(I have seen 'Animated film' mean 'cartoon' so perhaps 'film' could also mean 'cartoon')
'box containing old' is the wordplay.
I cannot really see how this works, but
'old' could be 'o' (common abbreviation eg in OE for Old English) and 'o' is located in the answer.
'box' could be 'carton' (carton is a kind of box) and 'carton' is present in the leftover letters.
This accounts for all the letters.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for cartoon that I've seen before include "Comic drawing" , "Humorous drawing or film of a succession of them" , "Witty drawing" , "eg 'The Simpsons'" , "could make one laugh" .)