Sticks marked off in units (6)
I believe the answer is:
rulers
'sticks marked off in units' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for rulers that I've seen before include "Drawing sticks or monarchs" , "Sticks marked off in units" , "Those in power" , "Strip with straight edge to measure" , "Sovereigns etc" .)