In Donegal a Moonie mission (5)
I believe the answer is:
alamo
'mission' is the definition.
(famous mission and siege from the Texas revolution)
'in donegal a moonie' is the wordplay.
'in' indicates the answer is hidden within the clue.
'ALAMO' can be found hidden inside 'donegal a moonie'.
(Other definitions for alamo that I've seen before include "Where Davy Crockett died, 1836" , "Famous battle in Texas and John Wayne film" , "Site of 1836 massacre in Texas" , "historical Franciscan mission" , "ill-fated mission" .)