Discrimination in America is set in stone (6)
I believe the answer is:
ageism
'discrimination' is the definition.
(I know that ageism is a type of discrimination)
'america is set in stone' is the wordplay.
'america' becomes 'a' (common abbreviation eg in USA).
'set in' means one lot of letters goes inside another.
'stone' becomes 'gem' (gemstone).
'is' put into 'gem' is 'geism'.
'a'+'geism'='AGEISM'
'in' is the link.
(Other definitions for ageism that I've seen before include "Prejudice based on advanced years" , "Prejudice in relation to advanced years" , "Discrimination on the basis of the length of one's life" , "Prejudice on grounds of advanced years" , "Bad attitude to the grey" .)