Volunteers to take a rum cocktail - twitch becomes shocking (9)
I believe the answer is:
traumatic
'shocking' is the definition.
(something traumatic is shocking)
'volunteers to take a rum cocktail twitch' is the wordplay.
'volunteers' becomes 'TA' (abbreviation for Territorial Army).
'to take' is an insertion indicator (inserted letters are taken in).
'cocktail' indicates an anagram.
'twitch' becomes 'tic' (tic is a kind of twitch).
'a'+'rum'='arum'
'arum' is an anagram of 'raum'.
'ta' enclosing 'raum' is 'trauma'.
'trauma'+'tic'='TRAUMATIC'
'becomes' acts as a link.
(Other definitions for traumatic that I've seen before include "Deeply disturbing" , "Psychologically painful" , "Psychologically damaging" , "Deeply distressing" , "Direly shocking" .)