Unit of length, one twelfth of a foot (4)
I believe the answer is:
inch
This clue is a double definition.
'unit of length' is the first definition.
(I've seen this before)
'one twelfth of a foot' is the second definition.
(I've seen this in another clue)
(Other definitions for inch that I've seen before include "... or not so long < , Celtic island" , "Measure of length" , "Edge slowly" , "Subdivision of 14 down" , "Imperial measure" .)