Councillor alarmed has to change name (8)
I believe the answer is:
alderman
'councillor' is the definition.
(alderman is a kind of councillor)
'alarmed has to change name' is the wordplay.
'has to change' indicates an anagram.
'name' becomes 'n' (common abbreviation e.g. on forms).
'alarmed' with letters rearranged gives 'alderma'.
'alderma'+'n'='ALDERMAN'
(Other definitions for alderman that I've seen before include "superior member of county council" , "In England and Wales, a former senior local councillor" , "Bigwig" , "Member of a local legislative body" , "Local politician" .)