One who leaves willingly (8)
I believe the answer is:
testator
'one who leaves willingly' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
This is all the clue.
Can you help me to learn more?
(Other definitions for testator that I've seen before include "one may need solicitor" , "A man who has made a will" , "Will signatory" , "Person who has made a will" , "Will provider" .)