Volunteers express disapproval — that’s not acceptable (5)
I believe the answer is:
taboo
'not acceptable' is the definition.
(something taboo is unacceptable)
'volunteers express disapproval' is the wordplay.
'volunteers' becomes 'TA' (abbreviation for Territorial Army).
'express disapproval' becomes 'boo' (as in a crowd booing).
'ta'+'boo'='TABOO'
'that's' is the link.
(Other definitions for taboo that I've seen before include "Socially prohibited" , "By religion or custom, not to be done or touched" , "Beyond the pale" , "Not allowed, forbidden, proscribed" , "Ritual prohibition" .)