A reason to slow for old sap (5)
I believe the answer is:
amber
'sap' is the definition.
The answer and definition can be both substances as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for amber that I've seen before include "Dark yellow" , "Yellowish-brown colour or resin" , "gives warning" , "warning signal" , "Cautionary colour" .)