In Donegal, a monastery and mission (5)
I believe the answer is:
alamo
'mission' is the definition.
(famous mission and siege from the Texas revolution)
'in donegal a monastery' is the wordplay.
'in' indicates the answer is hidden within the clue.
'ALAMO' is hidden in the letters of 'donegal a monastery'.
'and' acts as a link.
(Other definitions for alamo that I've seen before include "Texas mission besieged and captured by Mexican forces in 1836" , "Davy Crockett's last stand" , "Fort in San Antonio, once famously besieged" , "Texas fort, scene of famous siege by Mexican army" , "The answer is a place of siege" .)