Ruler with clear advantage (12)

I believe the answer is:

straightedge

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'ruler' is the definition.
(thesaurus)

'clear advantage' is the wordplay.
'clear' becomes 'straight' (I can't justify this - if you can you should believe this answer much more).
'advantage' becomes 'edge' (having the edge is having an advantage).
'straight'+'edge'='STRAIGHTEDGE'

'with' acts as a link.

Can you help me to learn more?

(Other definitions for straightedge that I've seen before include "rule" , "Not indulging in drug-taking" .)

I've seen this clue in The Irish Times.

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