Sliding round top (3)

I believe the answer is:

lid

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

'top' is the definition.
(I know that lid is a type of top)

'sliding' is the wordplay.
'LID' is hidden within the letters.
But, I am unsure how this is indicated.

'round' is the link.

Can you help me to learn more?

(Other definitions for lid that I've seen before include "Bottle or jar top" , "Hat" , "Top for bottle or eye protector" , "Cap or top for container" , "Covering" .)

I've seen this clue in The Mirror.

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