The most common and most likely answer to this crossword clue is the 11 letter word PROHIBITION
I've seen this crossword clue in Independent
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More clues leading to the result PROHIBITION
Buddy explains!
The straight part
We know that "It might herald a long dry period" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case it might herald a long dry period = prohibition.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 PROHIBITION mean?
PROHIBITION noun- An act of prohibiting, forbidding, disallowing, or proscribing something.
- A law prohibiting the manufacture or sale of alcohol.
- A period of time when specific socially disapproved consumables are considered controlled substances.