Offensive ass may transmit it through biting (7)
I believe the answer is:
tetanus
'may transmit it through biting' is the definition.
(tetanus can be spread by biting)
'offensive ass' is the wordplay.
'offensive' becomes 'tet' (Tet Offensive, Vietnam War military campaign).
'ass' becomes 'anus' (ass can mean the anus or buttocks).
'tet'+'anus'='TETANUS'
(Other definitions for tetanus that I've seen before include "Bacterial disease" , "Disease known as lockjaw from rabid dog bite" , "Disease causing muscular spasms" , "Infection of the central nervous system" , "Disease causing muscle spasms" .)