Backed some pro-Malaysian mission (5)
I believe the answer is:
alamo
'mission' is the definition.
(famous mission and siege from the Texas revolution)
'backed some pro-malaysian' is the wordplay.
'backed' says the letters should be written backwards.
'some' indicates a hidden word (some of the parts of the word are used).
'omala' is hidden in the letters of 'pro malaysian'.
'omala' back-to-front is 'ALAMO'.
(Other definitions for alamo that I've seen before include "Scene of massacre in Texas" , "historical Franciscan mission" , "Site of a Texas siege, 1836" , "Where Davy Crockett met his end" , "national monument" .)