Crossword Genius

Not a subject needed by most schoolchildren (5)


I believe the answer is:


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'subject needed by most schoolchildren' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Maybe they are linked in a way I don't understand?

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for ruler that I've seen before include "Monarch; measurer" , "king perhaps" , "Monarch or measure" , "Queen maybe" , "liner?" .)

I've seen this clue in The Globe and Mail.
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