Mostly offensive, wrapped in offensive label (6)
I believe the answer is:
ticket
'label' is the definition.
(thesaurus)
'mostly offensive wrapped in offensive' is the wordplay.
'mostly offensive' becomes 'ick' (I can't explain this - if you can you should believe this answer much more).
'wrapped in' is an insertion indicator.
'offensive' becomes 'tet' (Tet Offensive, Vietnam War military campaign).
'ick' put within 'tet' is 'TICKET'.
Can you help me to learn more?
(Other definitions for ticket that I've seen before include "It entitles the bearer, to enter perhaps" , "Buy one for admittance" , "Authority for entry, etc" , "lottery entry" , "Authorisation for travel, theatre etc" .)