The most common and most likely answer to this crossword clue is the 5 plus 2 plus 6 letter word REIGN OF TERROR
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More clues leading to the result REIGN OF TERROR
- Referring to or concerning a period of brutal oppression.
- Order given to ignore boner under newspaper — a dreadful rule!
- Referring to revolutionary, if not horrific period of history
- Loony fringe to slip in revolutionary period
- Repressive period referring to revolutionary men
- It was frightful, as a rule
- Referring to comic or tragic chapter in French history
Buddy explains!

The straight part
We know that "revolutionary period" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case revolutionary period = reign of terror.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.