One leaving land, perhaps, with river on a hill (8)
I believe the answer is:
testator
'one leaving land perhaps' is the definition.
(a testator leaves things in their will)
'river on a hill' is the wordplay.
'river' becomes 'test' (the River Test).
'on' says to put letters next to each other.
'hill' becomes 'tor' (tor is a kind of hill).
'test'+'a'+'tor'='TESTATOR'
'with' acts as a link.
(Other definitions for testator that I've seen before include "Person making a will" , "Person who has made a will" , "Will provider" , "Attestor (anag.)" , "One willing" .)