In Bengal a momentous mission (5)
I believe the answer is:
alamo
'mission' is the definition.
(famous mission and siege from the Texas revolution)
'in bengal a momentous' is the wordplay.
'in' indicates the answer is hidden within the clue.
'ALAMO' is hidden in the letters of 'bengal a momentous'.
(Other definitions for alamo that I've seen before include "Texas mission besieged and captured by Mexican forces in 1836" , "don't forget it!" , "Site of 1836 massacre in Texas" , "Where Davy Crockett met his end" , "Where Jim Bowie died" .)