Fast runner (8)
I believe the answer is:
sprinter
'fast runner' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for sprinter that I've seen before include "One moves quickly" , "Usain Bolt, for instance" , "He runs as fast as he can over short distance" , "speedy runner" , "one in the 100 metres" .)