Sadly cut short second mission (5)
I believe the answer is:
alamo
'mission' is the definition.
(famous mission and siege from the Texas revolution)
'sadly cut short second' is the wordplay.
'sadly' becomes 'alas' ('alas' is used to express regret).
'cut short' means to remove the last letter.
'second' becomes 'mo' (both are short periods of time).
'alas' with its final letter taken away is 'ala'.
'ala'+'mo'='ALAMO'
(Other definitions for alamo that I've seen before include "don't forget it!" , "The --, Texan mission besieged in 1836" , "Texas fort famed for defence against Mexican army" , "historical Franciscan mission" , "ill-fated mission" .)