Crossword Genius

Is one of these attached for part postage? (4)

Ross

I believe the answer is:

tags

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

'is' is the definition.
Although both the answer and definition are verbs in their -s form, I don't understand how they can define each other.

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for tags that I've seen before include "They are able to identify" , "trite Latin quotations" , "Labels; quotations" .)

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