Cryptic crossword clue
Alien conquest turned out to be significant 13

The most common and most likely answer to this crossword clue is the 13 letter word CONSEQUENTIAL

I've seen this crossword clue in Financial Times
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Buddy explains!

The straight part
We know that "significant" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case significant = consequential.
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 CONSEQUENTIAL mean?

CONSEQUENTIAL adjective
  1. Following as a result.
  2. Having significant consequences; of importance.
  3. Important or significant.
  4. (of a person) Self-important.

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