Unproductive nobleman, by the sound of it (6)
I believe the answer is:
barren
'unproductive' is the definition.
('barren' can be a synonym of 'unproductive')
'nobleman by the sound of it' is the wordplay.
'nobleman' becomes 'baron' (baron is a kind of nobleman).
'by the sound of it' shows a homophone (sound like).
'baron' is a homophone of 'BARREN'.
(Other definitions for barren that I've seen before include "Arid of unfertile" , "Not fertile or productive" , "Unproductive, arid" , "Arid, infertile" , "Producing little vegetation" .)