In Bhopal a momentous mission (5)
I believe the answer is:
alamo
'mission' is the definition.
(famous mission and siege from the Texas revolution)
'in bhopal a momentous' is the wordplay.
'in' says the answer is hidden in the clue.
'ALAMO' can be found hidden inside 'bhopal a momentous'.
(Other definitions for alamo that I've seen before include "Historic Texas mission station" , "ill-fated mission" , "Texas mission taken by Mexican forces in 1836" , "Famous battle in Texas launched by the Mexican army" , "Fort in San Antonio, once famously besieged" .)