Plain green ties needing to be done up (9)
I believe the answer is:
serengeti
'plain' is the definition.
(I know that Serengeti is a plain)
'green ties needing to be done up' is the wordplay.
'needing to be done up' indicates an anagram (letters improved or done up).
'green'+'ties'='greenties'
'greenties' with letters rearranged gives 'SERENGETI'.
(Other definitions for serengeti that I've seen before include "Tanzanian national park area" , "Tiger seen in Tanzania's great National Park" , "Green ties (anag.)" , "area of Tanzania" , "region in Tanzania" .)