Calculator shows playground etiquette (5,4)
I believe the answer is:
slide rule
'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" .)