Humorous drawing of box containing nothing (7)
I believe the answer is:
cartoon
'humorous drawing' is the definition.
(I've seen this before)
I can't explain the remainder of the clue.
(Other definitions for cartoon that I've seen before include "Amusing or satirical drawing" , "Jokey drawing" , "Humorous sketch" , "Animated drawing" , "eg 'Tom & Jerry'" .)