One running off after second runner (8)
I believe the answer is:
sprinter
'runner' is the definition.
(I know that sprinter is a type of runner)
'one running off after second' is the wordplay.
'one running off' becomes 'printer' (someone printing or running things off).
'after' means one lot of letters go next to another.
'second' becomes 's' (common abbreviation).
'printer' after 's' is 'SPRINTER'.
(Other definitions for sprinter that I've seen before include "one in the 100 metres" , "He runs as fast as he can over short distance" , "speedy runner" , "Short-distance runner" , "One moves quickly" .)