The most common and most likely answer to this crossword clue is the 9 letter word IMPELLING
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The straight part
We know that "Increasing pressure" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case increasing pressure = impelling.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 IMPELLING mean?
IMPELLING verb- To urge a person; to press on; to incite to action or motion via intrinsic motivation.
- To drive forward; to propel an object, to provide an impetus for motion or action.