Ethnic American spies infiltrating opposing sides (6)
I believe the answer is:
racial
'ethnic' is the definition.
(I've seen this before)
'american spies infiltrating opposing sides' is the wordplay.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'spies' becomes 'CIA' (Central Intelligence Agency).
'infiltrating' indicates putting letters inside.
'opposing sides' becomes 'RL' (right and left).
'a'+'cia'='acia'
'acia' placed into 'rl' is 'RACIAL'.
(Other definitions for racial that I've seen before include "Related to genetically different groups of people" , "Relating to distinctive human group" , "Of particular people or colour, as with prejudice" , "Relating to differences between nationalities" , "Relating to ethnicity" .)