Opposition to mixed ancestries (10)
I believe the answer is:
resistance
'opposition' is the definition.
(I know that opposition can be written as resistance)
'mixed ancestries' is the wordplay.
'mixed' indicates anagramming the letters.
'ancestries' is an anagram of 'RESISTANCE'.
'to' acts as a link.
(Other definitions for resistance that I've seen before include "Rebels" , "immunity" , "fighters in wartime" , "Withstanding" , "defiance" .)