US city, being all hippie, had changed (12)
I believe the answer is:
philadelphia
'us city' is the definition.
(I've seen this before)
'all hippie had changed' is the wordplay.
'changed' is an anagram indicator.
'all'+'hippie'+'had'='allhippiehad'
'allhippiehad' with letters rearranged gives 'PHILADELPHIA'.
'being' is the link.
(Other definitions for philadelphia that I've seen before include "US Quaker city, former capital" , "City of Brotherly Love" , "Pennsylvania city" , "City where the American Declaration of Independence was signed" , "US federal capital, 1790-1800" .)