Passage parallel to a diagram (9)
I believe the answer is:
paragraph
'passage' is the definition.
(I've seen this before)
'parallel to a diagram' is the wordplay.
'parallel to' becomes 'para' (I can't justify this - if you can you should believe this answer much more).
'a diagram' becomes 'graph' (I've seen this in another clue).
'para'+'graph'='PARAGRAPH'
Can you help me to learn more?
(Other definitions for paragraph that I've seen before include "Chunk of text" , "Unit of text" , "short item" , "A group of connected sentences" , "Section, division of written piece" .)