The one who makes a will (8)
I believe the answer is:
testator
'the one who makes a will' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for testator that I've seen before include "Will provider" , "Person making a will" , "One willing" , "Person who has made a will" , "one may need solicitor" .)