A beat round historical old mission (5)
I believe the answer is:
alamo
'historical old mission' is the definition.
'Alamo' can be an answer for 'mission' (famous mission and siege from the Texas revolution). I'm not certain of the 'historical old' bit.
'a beat round' is the wordplay.
'beat' becomes 'lam' (I've seen this before).
'round' becomes 'o' (round can mean a round object).
'a'+'lam'+'o'='ALAMO'
Can you help me to learn more?
(Other definitions for alamo that I've seen before include "The answer is a place of siege" , "Famous battle site in Texas" , "Where Jim Bowie died" , "Site of a Texas siege, 1836" , "The red planet" .)