Shock volunteers drinking a rum cocktail (6)
I believe the answer is:
trauma
'shock' is the definition.
(thesaurus)
'volunteers drinking a rum cocktail' is the wordplay.
'volunteers' becomes 'ta' (abbreviation for Territorial Army).
'drinking' means one lot of letters goes inside another.
'cocktail' is an anagram indicator.
'a'+'rum'='arum'
'arum' with letters rearranged gives 'raum'.
'ta' going around 'raum' is 'TRAUMA'.
(Other definitions for trauma that I've seen before include "Psychological wound" , "Deeply distressing or disturbing experience" , "Bodily or mental shock" , "Emotional upset" , "Physical or mental shock" .)