Distressing twitch about to irritate (6)
I believe the answer is:
tragic
'distressing' is the definition.
(I've seen this before)
'twitch about to irritate' is the wordplay.
'twitch' becomes 'tic' (tic is a kind of twitch).
'about' indicates putting letters inside.
'to irritate' becomes 'rag' (ragging is a kind of irritating).
'tic' going around 'rag' is 'TRAGIC'.
(Other definitions for tragic that I've seen before include "Disastrous, appalling" , "Dreadful" , "Very sad; Schubert's 4th symphony nickname" , "Ill-fated" , "Unfortunate" .)