Problem in America backing hostile stance (6)
I believe the answer is:
animus
'hostile stance' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
'problem in america backing' is the wordplay.
'problem' becomes 'sum' (as in a mathematical problem).
'america' becomes 'a' (common abbreviation eg in USA).
'backing' shows that the letters should be reversed in order.
'sum'+'in'+'a'='sumina'
'sumina' back-to-front is 'ANIMUS'.
Can you help me to learn more?
(Other definitions for animus that I've seen before include "Bad blood" , "Hostility, ill-feeling" , "Hostility shown in speech or action" , "Ill-feeling; hatred" , "Hostile spirit in speech or action" .)