Mission accomplished, leave and move on, first of all (5)
I believe the answer is:
alamo
'mission' is the definition.
(famous mission and siege from the Texas revolution)
'accomplished leave and move on first of all' is the wordplay.
'first of all' says to take the initial letters.
The initial letters of 'accomplished leave and move on' is 'ALAMO'.
(Other definitions for alamo that I've seen before include "Texas mission besieged and captured by Mexican forces in 1836" , "US mission" , "Mission building in Texas, site of famous siege by Mexico" , "Where Jim Bowie died" , "Famous Texan mission" .)