One eighth of a mile (7)
I believe the answer is:
furlong
'one eighth of a mile' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for furlong that I've seen before include "Measure of length" , "One eighth of a mile" , "1/8th of a mile" , "Imperial unit of length, an eighth of a mile" , "One-eight of a mile" .)