Awfully long, awful massacre often starts battle (5)
I believe the answer is:
Here is my best explanation:
'battle' is the definition.
(famous mission and siege from the Texas revolution)
'awfully long awful massacre often starts' is the subsidiary indication.
'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 "Famous battle in Texas and John Wayne film", "Famous Texan mission", "Famous battle site in Texas", "The --, Texan mission besieged in 1836", "Site of famous battle in Texas", "Texas mission besieged and captured by Mexican forces in 1836", "historical Franciscan mission", "Davy Crockett's last stand", "national monument" and "Texas mission taken by Mexican forces in 1836".)