The governor presumably knows where to draw the line (5)
I believe the answer is:
ruler
'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" .)