Unusually green site is plain (9)
I believe the answer is:
serengeti
'is plain' is the definition.
(I've seen this before)
'unusually green site' is the wordplay.
'unusually' indicates anagramming the letters.
'green'+'site'='greensite'
'greensite' is an anagram of 'SERENGETI'.
(Other definitions for serengeti that I've seen before include "place to watch game" , "Tiger seen in Tanzania's great National Park" , "area of Tanzania" , "Huge plain in Tanzania" , "area of the plain" .)