Mishap as one day overwhelmed by stress (8)
I believe the answer is:
accident
'mishap' is the definition.
(I've seen this before)
'one day overwhelmed by stress' is the wordplay.
'one' becomes 'i' (Roman numeral).
'day' becomes 'd' (abbreviation for day).
'overwhelmed by' is an insertion indicator (inserted letters get absorbed or overwhelmed).
'stress' becomes 'accent' (accent can mean to emphasise or stress).
'i'+'d'='id'
'id' put within 'accent' is 'ACCIDENT'.
'as' is the link.
(Other definitions for accident that I've seen before include "Unexpected incident" , "Unexpected and unplanned event" , "Mishap, misadventure" , "disaster" , "Unforseen misfortune" .)