Cautionary signal - slope's beginning to be ignored (5)
I believe the answer is:
amber
'cautionary signal' is the definition.
(I've seen this before)
'slope's beginning to be ignored' is the wordplay.
'slope' becomes 'camber' (camber is a kind of slope).
'beginning to be ignored' means to remove the first letter.
'camber' with its first letter removed is 'AMBER'.
(Other definitions for amber that I've seen before include "first light" , "Neither red nor green?" , "Orange traffic light" , "The traffic-light between red and green" , "Dark yellow" .)