Grit held loosely - it brings traffic to a standstill (3,5)
I believe the answer is:
red light
'it brings traffic to a standstill' is the definition.
The definition suggests a singular noun which matches the answer.
'grit held loosely' is the wordplay.
'loosely' indicates an anagram (a looser version of the letters).
'grit'+'held'='gritheld'
'gritheld' with letters rearranged gives 'RED-LIGHT'.
Can you help me to learn more?
(Other definitions for red light that I've seen before include "Don't go through it!" , "Illumination indicating danger" , "Colourful district housing brothels" , "sleazy" , "Stop signal" .)