Third leg broken by stop signal (3,5)
I believe the answer is:
red light
'stop signal' is the definition.
(I've seen this before)
'third leg broken' is the wordplay.
'broken' indicates an anagram.
'third'+'leg'='thirdleg'
'thirdleg' anagrammed gives 'RED-LIGHT'.
'by' is the link.
(Other definitions for red light that I've seen before include "Indication of immoral area" , "Stop signal" , "such a district is vicious" , "Colourful district housing brothels" , "Warning signal" .)