It’s only a part, but about right (7)
I believe the answer is:
excerpt
'right' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
'it's only a part but' is the wordplay.
'it's only a' becomes 'r' (I can't justify this - if you can you should believe this answer much more).
'part' means one lot of letters goes inside another (the inserted letters split open or part the word).
'but' becomes 'except' (I've seen this before).
'r' inserted inside 'except' is 'EXCERPT'.
'about' acts as a link.
Can you help me to learn more?
(Other definitions for excerpt that I've seen before include "part of article, perhaps" , "bit of work" , "Just a few words" , "Passage taken from book etc" , "Piece from book etc" .)