Historic mission (5)
I believe the answer is:
alamo
'historic mission' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for alamo that I've seen before include "Texan battle" , "don't forget it!" , "Where Davy Crockett met his end" , "Famous Texan fortress" , "Davy Crockett's last stand" .)