Texas mission (5)
I believe the answer is:
alamo
'texas mission' is the definition.
(the Alamo Mission, fortress in Texas)
This is the entire clue.
(Other definitions for alamo that I've seen before include "Site of a Texas siege, 1836" , "The --, Texan mission besieged in 1836" , "Mexicans invested in Texas" , "Famous Texan mission" , "Famous battle in Texas launched by the Mexican army" .)