The most common and most likely answer to this crossword clue is the 9 letter word HETERODOX
I've seen this crossword clue in Financial Times
H
E
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The straight part
We know that "Dissident" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case dissident = heterodox.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 HETERODOX mean?
HETERODOX adjective- Of or pertaining to creeds, beliefs, or teachings, especially religious ones, that are different from orthodoxy, or the norm, but not sufficiently different to be called heretical.