Governor that needs to be straight (5)
I believe the answer is:
I believe this clue is a double definition.
'governor' is the first definition.
(I've seen this before)
'that needs to be straight' is the second definition.
(a ruler must be straight)
(Other definitions for ruler that I've seen before include "Monarch - drafting tool" , "Measuring strip" , "Straight edge for drawing lines" , "Potentate" , "One for drawing lines" .)