Mission from Alaska shows up (5)
I believe the answer is:
alamo
'mission' is the definition.
(famous mission and siege from the Texas revolution)
'from alaska shows up' is the wordplay.
'shows' says the answer is hidden in the clue.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'omala' can be found hidden inside 'from alaska'.
'omala' reversed gives 'ALAMO'.
(Other definitions for alamo that I've seen before include "Famous battle in Texas launched by the Mexican army" , "Famous battle in Texas and John Wayne film" , "Where Davy Crockett died, 1836" , "Texas mission besieged and captured by Mexican forces in 1836" , "Where Davy Crockett met his end" .)