If you can remember this then all will be forgotten! (7)
I believe the answer is:
nothing
'if you can remember this then all will be forgotten' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
This is the entire clue.
Can you help me to learn more?
(Other definitions for nothing that I've seen before include "Naught, nil, nix" , "Nought; of no value" , "Item of no importance" , "One of those sweet sayings" , "Something sweet" .)