Rupert is out of plate armour for the mission that was under siege in Texas (5)

I believe the answer is:

alamo

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'in texas' is the definition.
('Alamo' is part of 'texas')

I cannot understand how the remainder of the clue works.

(Other definitions for alamo that I've seen before include "Historic Texas mission station" , "US mission" , "Famous battle in Texas and John Wayne film" , "Scene of massacre in Texas" , "Texas fort famed for defence against Mexican army" .)

I've seen this clue in the Irish Times and the Irish Times (Crosaire).

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