Nobleman not producing results in speech (5)
I believe the answer is:
baron
'nobleman' is the definition.
(baron is a kind of nobleman)
'not producing results in speech' is the wordplay.
'not producing results' becomes 'barren' (barren means unproductive).
'in speech' indicates a 'sounds like' (homophone) clue.
'barren' sounds like 'BARON'.
(Other definitions for baron that I've seen before include "Minor noble" , "Minor peer" , "See nobleman" , "Minor nobleman" , "A lord; sounds like childless" .)