Unproductive noble, by the sound of it (6)
I believe the answer is:
barren
'unproductive' is the definition.
(synonyms)
'noble by the sound of it' is the wordplay.
'noble' becomes 'baron' (baron is a kind of noble).
'by the sound of it' indicates a 'sounds like' (homophone) clue.
'baron' sounds like 'BARREN'.
(Other definitions for barren that I've seen before include "Unable to reproduce, infertile" , "Incapable of sustaining life" , "Bleak, lifeless" , "Not producing vegetation or offspring" , "Unproductive of life" .)