Governor in something made to measure (5)
I believe the answer is:
ruler
I believe this is a double definition.
'governor' is the first definition.
(thesaurus)
'something made to measure' is the second definition.
'in' acts as a link.
(Other definitions for ruler that I've seen before include "Graduated straight-edge" , "one responsible for a line?" , "One in power" , "Head of state" , "Straight edge; monarch" .)