Additional house numbers? (7)
I believe the answer is:
codicil
'additional house numbers?' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
This is all the clue.
Can you help me to learn more?
(Other definitions for codicil that I've seen before include "Addition to a will" , "Portion added on to a will" , "Additional item appended to a will" , "Addendum to will" , "Will's rider" .)