Knowledgeable writer who wrote nonsense about study (7)
I believe the answer is:
learned
'knowledgeable writer who' is the definition.
'learned' can be an answer for 'knowledgeable' (I've seen this before). I am unsure of the rest of the definition.
'wrote nonsense about study' is the wordplay.
'wrote nonsense' becomes 'lear' (I can't explain this - if you can you should believe this answer much more).
'about' shows that the letters should be reversed in order.
'study' becomes 'den' (term for a study or office).
'den' backwards is 'ned'.
'lear'+'ned'='LEARNED'
Can you help me to learn more?
(Other definitions for learned that I've seen before include "Educated, scholarly" , "Having great knowledge" , "Wise, educated" , "Gained understanding" , "Knowledgeable - scholarly" .)