Prompt report of traffic jam? (7)
I believe the answer is:
autocue
'prompt' is the definition.
I can't explain the rest of the clue.
(Other definitions for autocue that I've seen before include "Newscaster's reading aid" , "Device displaying words to be read" , "Newsreader's prompt" , "Speaker's aid" , "Hidden screen showing the script to a speaker" .)