Official circle in army organisation (5)
I believe the answer is:
mayor
'official' is the definition.
(I have seen 'Municipal official' mean 'mayor' so perhaps 'official' could also mean 'mayor')
'circle in army organisation' is the wordplay.
'circle' becomes 'O' (O is a circle).
'in' indicates putting letters inside.
'organisation' indicates an anagram.
'army' anagrammed gives 'mayr'.
'o' inserted into 'mayr' is 'MAYOR'.
(Other definitions for mayor that I've seen before include "Elected town/city head" , "City leader" , "Civic official" , "Municipal leader" , "Civic leader" .)