America cunningly infiltrated by a revolutionary communist brotherhood (11)
I believe the answer is:
camaraderie
'brotherhood' is the definition.
(thesaurus)
'america cunningly infiltrated by a revolutionary communist' is the wordplay.
'cunningly' is an anagram indicator.
'infiltrated by' means one lot of letters goes inside another.
'revolutionary' is a reversal indicator.
'communist' becomes 'red' (red is a kind of communist).
'red' backwards is 'der'.
'america' with letters rearranged gives 'camarie'.
'a'+'der'='ader'
'camarie' going around 'ader' is 'CAMARADERIE'.
(Other definitions for camaraderie that I've seen before include "Friendly feeling" , "Friendly sociability" , "Main theme of Friends" , "Fellowship" , "Spirit of friendliness and trust" .)