One in race to destruction (4)
I believe the answer is:
ruin
'destruction' is the definition.
(ruin is a kind of destruction)
'one in race' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' is an insertion indicator.
'race' becomes 'run' (run is a kind of race**).
'i' going into 'run' is 'RUIN'.
'to' is the link.
(Other definitions for ruin that I've seen before include "Destroy; downfall" , "Destroy or bankrupt" , "financial disaster" , "dishonour" , "Disaster, collapse" .)