In general, a mournful site of battle (5)
I believe the answer is:
Here is my best explanation:
'battle' is the definition.
(famous mission and siege from the Texas revolution)
'in general a mournful site' is the subsidiary indication.
'in' says the answer is hidden in the clue.
'alamo' is hidden within 'general a mournful site'.
'of' is the link.
(Other definitions for alamo that I've seen before include "Mission building in Texas, site of famous siege by Mexico", "Massacre scene in Texas", "The --, Texan mission besieged in 1836", "Where Jim Bowie died", "US mission", "Texas site of 1836 siege", "Famous battle site in Texas", "Site of famous battle in Texas", "The --, historic mission in Texas" and "Davy Crockett's last stand".)