The most common and most likely answer to this crossword clue is the 8 letter word SAMIZDAT
I've seen this crossword clue in The Guardian
S
A
M
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Buddy explains!

The straight part
We know that "secret publishing" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case secret publishing = samizdat.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does SAMIZDAT mean?
SAMIZDAT noun- The secret copying and sharing of illegal publications, chiefly in the Soviet Union; underground publishing and its publications.
- A samizdat publication.