Not a subject needed by most schoolchildren (5)
I believe the answer is:
'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?" .)