Selected portion of former publication (7)
I believe the answer is:
'selected portion' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'former publication' is the wordplay.
'former' becomes 'ex' (prefix meaning former or onetime).
'publication' becomes 'tract' (I've seen this before).
'of' is the link.
Can you help me to learn more?
(Other definitions for extract that I've seen before include "Passage selected from a larger work" , "Extirpate" , "Take out with effort, say a tooth" , "Part" , "short passage from a book" .)