Generally how a straight edge is used (2,1,4)

I believe the answer is:

as a rule

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'generally' is the definition.
(I've seen this in another clue)

I don't understand how the rest of the clue works.

(Other definitions for as a rule that I've seen before include "Generally, normally" , "Mainly" , "Usually but not always" , "26 [NORMALLY]" , "Most of the time" .)

I've seen this clue in the Sydney Morning Herald.

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