Nobleman let off polymath (9)
I believe the answer is:
aristotle
'polymath' is the definition.
(I've seen this before)
'nobleman let off' is the wordplay.
'nobleman' becomes 'aristo' (I can't explain this - if you can you should believe this answer much more).
'off' indicates anagramming the letters.
'let' with letters rearranged gives 'tle'.
'aristo'+'tle'='ARISTOTLE'
Can you help me to learn more?
(Other definitions for aristotle that I've seen before include "noted thinker" , "Realist to be like great Greek philosopher" , "Greek philosopher (Poetics, Politics)" , "Greek philosopher, pupil of Plato, tutor of Alexander the Great" , "polymath" .)