Everyone breaking down upset, in US city (6)
I believe the answer is:
dallas
'us city' is the definition.
(I've seen this before)
'everyone breaking down upset' is the wordplay.
'everyone' becomes 'all' (I've seen this before).
'breaking' indicates putting letters inside.
'down' becomes 'sad' (I've seen this before**).
'upset' is a reversal indicator.
'all' inserted within 'sad' is 'sallad'.
'sallad' written backwards gives 'DALLAS'.
'in' acts as a link.
(Other definitions for dallas that I've seen before include "Texas city - old soap" , "American city where JFK was killed" , "City in the heart of Texan oil fields" , "Mysteriously, all sad in the US city" , "former TV series" .)