US city’s backed argument for amalgamation (9)
I believe the answer is:
synthesis
'amalgamation' is the definition.
(thesaurus)
'us city's backed argument' is the wordplay.
'us city's' becomes 'NY's' (New York).
'backed' is a reversal indicator.
'argument' becomes 'thesis' (a thesis is an argument or proposal).
'nys' backwards is 'syn'.
'syn'+'thesis'='SYNTHESIS'
'for' is the link.
(Other definitions for synthesis that I've seen before include "Process of bringing together to form a whole" , "Result of combining" , "Fusion" , "In shy sets (anag.)" , "Chemical or dialectical blending process" .)