The most common and most likely answer to this crossword clue is the 9 letter word GOVERNING
I've seen this crossword clue in Independent
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More clues leading to the result GOVERNING
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The straight part
We know that "a rule?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a rule? = governing.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 GOVERNING mean?
GOVERNING verb- To make and administer the public policy and affairs of; to exercise sovereign authority in.
- To control the actions or behavior of; to keep under control; to restrain.
- To exercise a deciding or determining influence on.
- To control the speed, flow etc. of; to regulate.
- To exercise political authority; to run a government.
- To have or exercise a determining influence.
- (grammar) To require that a certain preposition, grammatical case, etc. be used with a word; sometimes used synonymously with collocate.