Lightbulb, in cartoons (4)
I believe the answer is:
idea
'lightbulb in cartoons' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for idea that I've seen before include "Fancy" , "Intention" , "Notion, image" , "Thought or notion (4)" , "Brainwave" .)