Box contains old film (7)
I believe the answer is:
'film' is the definition.
(cartoon is a kind of film)
'box contains old' is the wordplay.
I cannot quite see how this works, but
'old' could be 'o' (common abbreviation eg in OE for Old English) and 'o' is present in the answer.
'box' could be 'carton' (carton is a kind of box) and 'carton' is found in the remaining letters.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for cartoon that I've seen before include "Humorous drawing or film of a succession of them" , "Comic drawing" , "Comical drawing" , "Comic strip" , "Humorous sketch" .)