The downfall of one in a sprint (4)
I believe the answer is:
ruin
'the downfall' is the definition.
(I've seen this before)
'one in a sprint' is the wordplay.
'one' becomes 'I' (Roman numeral).
'in' is an insertion indicator.
'a sprint' becomes 'run' (I've seen this in another clue).
'i' placed inside 'run' is 'RUIN'.
'of' acts as a link.
(Other definitions for ruin that I've seen before include "Destroy; downfall" , "Building remains" , "failure" , "Dilapidated building" , "Scotch" .)