The most common and most likely answer to this crossword clue is the 7 letter word HERETIC
I've seen this crossword clue in Independent
H
E
R
E
T
I
C
More clues leading to the result HERETIC
- Charlie supporting either crackpot or dissident
- Non-orthodox call for attention gets habitual response
- Turning back, some incite rehabilitated dissenter
- One doesn’t believe the woman rejected quote
- . Dissident in this place has mark of approval curtailed
- Free-thinker’s name raised by that woman
- Nietzsche reticently welcoming dissident
- Dissident woman’s name seen in the mirror
- Dissenter causing present convulsion
- In hi-tec broadcast, Brenda’s no Defender of the Faith
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 = heretic.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 HERETIC mean?
HERETIC noun- Someone who believes contrary to the fundamental tenets of a religion they claim to belong to.
- Someone who does not conform to generally accepted beliefs or practices
- Heretical; of or pertaining to heresy or heretics.