Awfully long, awful massacre often starts battle (5)
I believe the answer is:
alamo
'battle' is the definition.
(famous mission and siege from the Texas revolution)
'awfully long awful massacre often starts' is the wordplay.
'starts' suggests taking the first letters.
The first letters of 'awfully long awful massacre often' is 'ALAMO'.
(Other definitions for alamo that I've seen before include "Where Davy Crockett met his end" , "Famous battle in Texas and John Wayne film" , "The red planet" , "don't forget it!" , "Mission building in Texas, site of famous siege by Mexico" .)