Destroy one entering race (4)
I believe the answer is:
ruin
'destroy' is the definition.
('ruin' can be a synonym of 'destroy')
'one entering race' is the wordplay.
'one' becomes 'i' (Roman numeral).
'entering' means one lot of letters goes inside another.
'race' becomes 'run' (run is a kind of race**).
'i' inserted inside 'run' is 'RUIN'.
(Other definitions for ruin that I've seen before include "Utter destruction or downfall" , "fall" , "Disaster, collapse" , "Destroy completely" , "Wreck" .)