Measure leader (5)
I believe the answer is:
ruler
I believe this clue is a double definition.
'measure' is the first definition.
(I know that ruler is a type of measure)
'leader' is the second definition.
(I've seen this before)
(Other definitions for ruler that I've seen before include "Commander" , "Graduated straight-edge" , "person holding sway?" , "It helps to draw straight lines" , "Measuring device" .)