Calculator shows playground etiquette (5,4)
I believe the answer is:
slide rule
'calculator' is the definition.
(I've seen this before)
I cannot understand how the remainder of the clue works.
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" .)