One running second machine (8)
I believe the answer is:
sprinter
'one running' is the definition.
(I've seen this before)
'second machine' is the wordplay.
I cannot quite understand how this works, but
'second' could be 's' (common abbreviation) and 's' is present in the answer.
'machine' could be 'printer' (printer is a kind of machine) and 'printer' is found in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for sprinter that I've seen before include "One moves quickly" , "one running" , "speedy runner" , "Athlete" , "one in the 100 metres" .)