About vehicle overturned by speedster (5)
I believe the answer is:
racer
'speedster' is the definition.
(I've seen this in another clue)
'about vehicle overturned' is the wordplay.
'about' becomes 're' ('re' means regarding or about).
'vehicle' becomes 'car' (car is a kind of vehicle).
'overturned' is a reversal indicator.
're'+'car'='recar'
'recar' back-to-front is 'RACER'.
'by' acts as a link.
(Other definitions for racer that I've seen before include "sportsman" , "One car" , "Fast competitive car or animal" , "someone driving very fast?" , "Fast-moving animal" .)