The most common and most likely answer to this crossword clue is the 12 letter word PROTECTORATE
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The straight part
We know that "That was it" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case that was it = protectorate.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 PROTECTORATE mean?
PROTECTORATE noun- Government by a protector; especially, the government of England, Scotland, and Ireland by Oliver Cromwell.
- The authority assumed by a superior power over an inferior or a dependent one, whereby the former protects the latter from invasion and shares in the management of its affairs but the protected state retains its nominal sovereignty.
- An autonomous territory that is protected diplomatically or militarily against third parties by a stronger state or entity.