Wreck one in race (4)
I believe the answer is:
ruin
'wreck' is the definition.
(I've seen this before)
'one in race' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' indicates putting letters inside.
'race' becomes 'run' (running is a kind of racing).
'i' inserted into 'run' is 'RUIN'.
(Other definitions for ruin that I've seen before include "Devastate" , "Complete collapse or destruction" , "Destruction or bankruptcy" , "Catastrophe" , "Destory or bankrupt" .)