A quick temper for the US’s terrible (5,4)
I believe the answer is:
short fuse
'a quick temper' is the definition.
(I've seen this before)
'for the us's terrible' is the wordplay.
'terrible' indicates an anagram.
'for'+'the'+'uss'='fortheuss'
'fortheuss' with letters rearranged gives 'SHORT FUSE'.
(Other definitions for short fuse that I've seen before include "Quick temper (little piece of wire?)" , "irascibility" , "On which one is quick to lose one's temper" .)