Fifty savages destroyed US city (3,5)
I believe the answer is:
las vegas
'us city' is the definition.
(I've seen this before)
'fifty savages destroyed' is the wordplay.
'fifty' becomes 'l' (roman numeral).
'destroyed' indicates an anagram.
'savages' anagrammed gives 'asvegas'.
'l'+'asvegas'='LAS VEGAS'
(Other definitions for las vegas that I've seen before include "US resort - with 23d" , "US gambling centre" , "city surrounded by desert" , "'City famous for gambling and excess (3,5)'" , "Nevada city famed for casinos" .)