One willing in trial on a hill (8)
I believe the answer is:
testator
'one willing' is the definition.
(person making a will)
'trial on a hill' is the wordplay.
'trial' becomes 'test' (synonyms).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'hill' becomes 'tor' (tor is a kind of hill).
'test'+'a'+'tor'='TESTATOR'
'in' acts as a link.
(Other definitions for testator that I've seen before include "Will provider" , "One making a will" , "A man who has made a will" , "One willing" , "Person who has made a will" .)