Chart leaving room for cautionary sign (5)
I believe the answer is:
amber
'sign' is the definition.
(I know that amber is a type of visual signal)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for amber that I've seen before include "Prepare-to-stop signal" , "Follilised resin" , "A yellowy resin" , "forever written about" , "Traffic light colour" .)