Crossword Genius

Some shaky tremors in the underground, abroad (5)

Ross

I believe the answer is:

metro

I'm a little stuck... Click here to teach me more about this clue!

'some shaky tremors in' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for metro that I've seen before include "French underground" , "what moves many Parisians" , "Paris underground rail system" , "The underground railway system in Paris" , "Parisian underground system" .)

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