Crossword Genius

Calculator shows playground etiquette (5,4)

Ross

I believe the answer is:

slide rule

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

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

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for slide rule that I've seen before include "Old calculating device" , "old computer" , "Instrument for rapid calculations" , "Old calculator" , "Obsolete calculator" .)

I've seen this clue in The Mirror.
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