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