Mishap I ultimately witnessed, in a manner of speaking (8)
I believe the answer is:
accident
'mishap' is the definition.
(thesaurus)
'i ultimately witnessed in a manner of speaking' is the wordplay.
'ultimately' indicates one should take the final letters.
'in' is an insertion indicator.
'a manner of speaking' becomes 'accent' (accent is a kind of manner of speaking**).
The final letter of 'witnessed' is 'd'.
'i'+'d'='id'
'id' put into 'accent' is 'ACCIDENT'.
(Other definitions for accident that I've seen before include "disaster" , "Unfortunate unintentional incident" , "Chance event with no apparent cause" , "Pile-up" , "Unexpected and unplanned event" .)