It states where what is left should go (4)
I believe the answer is:
will
This clue could be a double definition.
'it states where what is left should' is the first definition.
The definition suggests a singular noun which matches the answer.
'go' is the second definition.
The answer and definition can be both to do with fundamental existence as well as being verbs in their base form.
Maybe you can see a link between them that I can't see?
Can you help me to learn more?
(Other definitions for will that I've seen before include "Record of final wishes and testament" , "Intention; legal document" , "Shakespeare < 's intention to leave something" , "Final disposition" , "Intention, desire" .)