Long race for a runner (4)
I believe the answer is:
mile
'long race for a runner' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for mile that I've seen before include "'A miss is as good as a ....'" , "Stretch of road" , "1.61km" , "Eight furlongs" , "Unit of distance" .)