Selected portion of former publication (7)
I believe the answer is:
extract
'selected portion' is the definition.
Although both the answer and definition are singular nouns, I cannot 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).
'ex'+'tract'='EXTRACT'
'of' acts as a 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" .)