Top US city backed a source of wool (6)
I believe the answer is:
alpaca
'source of wool' is the definition.
(I know that alpaca this taxon is source of alpaca fiber)
'top us city backed a' is the wordplay.
'top' becomes 'cap' (both can mean to beat or outdo).
'us city' becomes 'LA' (short for Los Angeles).
'backed' is a reversal indicator.
'cap'+'la'='capla'
'capla' written backwards gives 'alpac'.
'alpac'+'a'='ALPACA'
(Other definitions for alpaca that I've seen before include "Long-haired mammal related to the llama" , "Type of wool" , "Break down (food)" , "Llama relative, valued for it's wool" , "S American mammal related to the llama" .)