Unproductive peer, by the sound of it (6)
I believe the answer is:
barren
'unproductive' is the definition.
(synonyms)
'peer by the sound of it' is the wordplay.
'peer' becomes 'baron' (baron is a kind of peer).
'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 "Unproductive of life" , "Not fertile, unproductive" , "Not producing vegetation or offspring" , "Bare and bleak" , "'Desolate, fallow (6)'" .)