Crossword Genius

Titanic reportedly swarming with small creatures (4)

Ross

I believe the answer is:

mity

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'with small creatures' is the definition.
The definition suggests a singular noun which matches the answer.

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for mity that I've seen before include "With many little creatures" , "Infested with arachnids" , "Infested with horrible things that are tiny" .)

I've seen this clue in The Sunday Times and The Times.
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