Twelfth of a foot (4)
I believe the answer is:
inch
'twelfth of a foot' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for inch that I've seen before include "A very little" , "The island is very small in length" , "Creep towards < island" , "2.54cm" , "... or not so long < , Celtic island" .)