The most common and most likely answer to this crossword clue is the 15 letter word TOTALITARIANISM
I've seen this crossword clue in Financial Times
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More clues leading to the result TOTALITARIANISM
- Limitation as art reworked in autocratic approach
- Horrible attritional aims in dictatorship?
- Ordered militants to adopt song in advocacy of tyranny
- Revolutionary 1 24 or not, Mao and Stalin engaged in it
- Young child settled heretical belief in tyranny
- Iron fist of Stalin, taking aim at riot violently
- School so martial, ain't it an autocratic system?
- Limitation a tsar abandoned in dictatorship
- A militant riots: a radical dictatorship
Buddy explains!

The straight part
We know that "advocacy of tyranny" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case advocacy of tyranny = totalitarianism.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 TOTALITARIANISM mean?
TOTALITARIANISM noun- A system of government in which the people have virtually no authority and the state wields absolute control, for example, a dictatorship.