Said noble is without hope of heir (6)
I believe the answer is:
barren
'without hope of heir' is the definition.
(barren can mean infertile)
'said noble' is the wordplay.
'said' shows a homophone (sound like).
'noble' becomes 'baron' (baron is a kind of noble).
'baron' is a homophone of 'BARREN'.
'is' is the link.
(Other definitions for barren that I've seen before include "Bleak, lifeless" , "Producing little vegetation" , "Empty" , "Unable to reproduce, infertile" , "Bare and bleak" .)