Hideous thing witnessed, I'm exasperated and traumatised initially (5)
I believe the answer is:
sight
'hideous thing witnessed' is the definition.
'i'm exasperated and traumatised initially' is the wordplay.
'i'm exasperated' becomes 'sigh'.
'and' means one lot of letters go next to another.
'initially' suggests taking the first letters.
The first letter of 'traumatised' is 't'.
'sigh'+'t'='SIGHT'
(Other definitions for sight that I've seen before include "Sense" , "Power of vision" , "Glimpse" , "Vision, ability to see" , "Looking" .)