Severe traffic jam (8)
I believe the answer is:
gridlock
'severe traffic jam' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for gridlock that I've seen before include "Extensive traffic jam" , "Traffic jam at intersections" , "Traffic snarl-up" , "road problem" , "Traffic jam where no movement is possible" .)