One trapped in race wreck (4)
I believe the answer is:
ruin
'wreck' is the definition.
(I've seen this before)
'one trapped in race' is the wordplay.
'one' becomes 'I' (Roman numeral).
'trapped in' indicates putting letters inside.
'race' becomes 'run' (run is a kind of race).
'i' put inside 'run' is 'RUIN'.
(Other definitions for ruin that I've seen before include "Destroy or bankrupt" , "Destroy completely" , "Spoil, impoverish" , "Wrecked building" , "Destruction or bankruptcy" .)