Crossword Genius

The governor presumably knows where to draw the line (5)

Ross

I believe the answer is:

ruler

I'm a little stuck... Click here to teach me more about this clue!

'the governor' is the definition.
(I've seen this before)

'presumably knows where to draw the line' is the wordplay.
I cannot quite see how this works, but
'draw' could be 'rule' (ruling is a kind of drawing) and 'rule' is found in the answer.
A single letter 'r' remains which might be clued in a way I don't see.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for ruler that I've seen before include "Source of line" , "person holding sway?" , "Measuring tool" , "Measuring stick" , "One in power" .)

I've seen this clue in the Evening Standard.
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