One will change? (7)
I believe the answer is:
codicil
This might be a double definition.
'one' is the first definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'will change?' is the second definition.
(I've seen this in another clue)
Can you help me to learn more?
(Other definitions for codicil that I've seen before include "Portion added on to a will" , "Modifier to will" , "Appendix to a will" , "Addendum to will" , "Will's rider" .)