Way to work on sadly curtailed mission (5)
I believe the answer is:
alamo
'mission' is the definition.
(famous mission and siege from the Texas revolution)
'way to work on sadly curtailed' is the wordplay.
'way to work' becomes 'mo' (I've seen this in other clues).
'on' means one lot of letters go next to another.
'sadly' becomes 'alas' ('alas' is used to express regret).
'curtailed' means to remove the last letter.
'alas' with its final letter taken away is 'ala'.
'mo' after 'ala' is 'ALAMO'.
(Other definitions for alamo that I've seen before include "Famous battle in Texas launched by the Mexican army" , "Where Jim Bowie died" , "Historic Texas mission station" , "Texas mission taken by Mexican forces in 1836" , "Mexicans invested in Texas" .)