Might, alternatively, become an official (5)
I believe the answer is:
mayor
'an official' is the definition.
(I have seen 'Municipal official' mean 'mayor' so perhaps 'official' could also mean 'mayor')
'might alternatively' is the wordplay.
'might' becomes 'may' (I've seen this in another clue).
'alternatively' becomes 'or'.
'may'+'or'='MAYOR'
'become' acts as a link.
(Other definitions for mayor that I've seen before include "Local government head" , "Town council head" , "Civic official" , "Johnson, say" , "Corporation head" .)