Stress about one day's mishap (8)
I believe the answer is:
accident
'mishap' is the definition.
(I've seen this before)
'stress about one day's' is the wordplay.
'stress' becomes 'accent' (synonyms).
'about' indicates putting letters inside.
'one' becomes 'I' (Roman numeral).
'day' becomes 'd' (abbreviation for day).
'i'+'d'='id'
'accent' enclosing 'id' is 'ACCIDENT'.
(Other definitions for accident that I've seen before include "Fortune" , "Calamity" , "disaster" , "Dance tic leading to mishap" , "Mishap, misadventure" .)