Crossword Genius

Rumpus when old books disappear from Ulster castle (5)

Ross

I believe the answer is:

storm

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'rumpus' is the definition.
(I've seen this before)

I cannot understand how the remainder of the clue works.

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(Other definitions for storm that I've seen before include "SECOND PART OF 1 DOWN" , "Squall" , "Atmospheric disturbance, assault (stronghold)," , "- drain; - cloud" , "rage" .)

I've seen this clue in The Times.
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