African runner's go in flash car (7)
I believe the answer is:
limpopo
'african runner's' is the definition.
(African river, something which runs)
'go in flash car' is the wordplay.
'go' becomes 'pop' (popping is a kind of going).
'in' is an insertion indicator.
'flash car' becomes 'limo' (limousine, a fancy car).
'pop' placed within 'limo' is 'LIMPOPO'.
(Other definitions for limpopo that I've seen before include "African river" , "River of SE Africa" , "River of South Africa" , "S. Africa/Zimbabwe border river" , "African runner" .)