Joyful noise supporting City (8)
I believe the answer is:
ecstatic
'joyful' is the definition.
(I've seen this before)
'noise supporting city' is the wordplay.
'noise' becomes 'static' (static is a kind of noise**).
'supporting' means one lot of letters go next to another (in a down clue, some letters go below others).
'city' becomes 'ec' (the City of London postcode).
'static' after 'ec' is 'ECSTATIC'.
(Other definitions for ecstatic that I've seen before include "Really overjoyed, gleeful" , "Emotionally exalted" , "In a state of great delight" , "Feeling immense joy" , "on top of the world" .)