Song about US city established in plain (6)
I believe the answer is:
ballad
'song' is the definition.
(I know that ballad is a type of song)
'us city established in plain' is the wordplay.
'us city' becomes 'LA' (Los Angeles).
'established in' indicates putting letters inside.
'plain' becomes 'bald' (both can mean unadorned).
'la' placed inside 'bald' is 'BALLAD'.
'about' is the link.
(Other definitions for ballad that I've seen before include "Narrative poem or song" , "Narrative song with a recurrent refrain" , "Folk song" , "Song type" , "Love song" .)