Finish last, beset by flat tire and engine failure, potentially fatal (4-11)

'potentially fatal' is the definition.
(something life-threatening might be fatal)

'finish last beset by flat tire and engine failure' is the wordplay.
'last' indicates one should take the final letters.
'beset by' is an insertion indicator ('beset' could historically mean 'cover').
'and' is a charade indicator (letters next to each other).
'failure' indicates anagramming the letters.
The final letter of 'finish' is 'h'.
'flat'+'tire'+'engine'='flattireengine'
'flattireengine' is an anagram of 'lifetreatening'.
'h' put within 'lifetreatening' is 'LIFE-THREATENING'.