Ruler unit (4)
I believe the answer is:
inch
'ruler unit' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for inch that I've seen before include "Small measurement unit of distance" , "Creep, edge" , "Advance slowly" , "2.45 cm" , "One of a 2 down in a foot" .)