Motorists won't be slow to fall into it (5,4)
I believe the answer is:
speed trap
'it' is the definition.
The definition suggests a singular noun which matches the answer.
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for speed trap that I've seen before include "something drivers might avoid" , "Hidden device" , "hazard for travellers in haste?" , "that slows motorist?" , "road safety measure" .)