Damage volunteers with a rum cocktail (6)
I believe the answer is:
trauma
'damage' is the definition.
(I know that trauma is a type of damage)
'volunteers with a rum cocktail' is the wordplay.
'volunteers' becomes 'TA' (abbreviation for Territorial Army).
'with' indicates putting letters inside.
'cocktail' indicates an anagram.
'a'+'rum'='arum'
'arum' anagrammed gives 'raum'.
'ta' going around 'raum' is 'TRAUMA'.
(Other definitions for trauma that I've seen before include "Deep distress; injury" , "Lasting shock or disturbance" , "dreadful shock!" , "Nervous shock" , "Bodily or mental shock" .)