Crossword Genius

An impression mud is rising, not permanently (2,7)

Ross

I believe the answer is:

ad interim

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'not permanently' is the definition.
('ad interim' is the opposite to 'permanently')

I don't understand the remainder of the clue.

Can you help me to learn more?

(Another definition for ad interim that I've seen is " in the meantime".)

I've seen this clue in The Times.
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