Revolutionary feat to have United Nations turning brown (8)
I believe the answer is:
chestnut
'brown' is the definition.
(I've seen this before)
'revolutionary feat to have united nations turning' is the wordplay.
I cannot really see how this works, but
'revolutionary' could be 'che' (Che Guevara the Marxist revolutionary) and 'che' is present in the answer.
'united' could be 'u' and 'u' is present in the answer.
This explanation may well be incorrect...
(Other definitions for chestnut that I've seen before include "Old story" , "not a cracker!" , "Golden to reddish brown colour" , "Sweet -; horse -; water -" , "Tree - reddish-brown" .)