A mob almost seizing city in battle (5)
I believe the answer is:
alamo
'battle' is the definition.
(famous mission and siege from the Texas revolution)
'a mob almost seizing city' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'seizing' means one lot of letters goes inside another.
'city' becomes 'LA' (Los Angeles).
'a'+'mob'='amob'
'amob' with its final letter removed is 'amo'.
'amo' placed around 'la' is 'ALAMO'.
'in' acts as a link.
(Other definitions for alamo that I've seen before include "Historic Texas mission station" , "historical Franciscan mission" , "US mission" , "Famous battle in Texas launched by the Mexican army" , "national monument" .)