The most common and most likely answer to this crossword clue is the 10 letter word PROTESTING
I've seen this crossword clue in The Guardian
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More clues leading to the result PROTESTING
Buddy explains!

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 = protesting.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 PROTESTING mean?
PROTESTING verb- To make a strong objection.
- To affirm (something).
- To object to.
- To call as a witness in affirming or denying, or to prove an affirmation; to appeal to.
- To make a solemn written declaration, in due form, on behalf of the holder, against all parties liable for any loss or damage to be sustained by non-acceptance or non-payment of (a bill or note). This should be made by a notary public, whose seal it is the usual practice to affix.
- To publish; to make known.
- Protestation.