Greenest resorts, with one in National Park (9)
I believe the answer is:
serengeti
'national park' is the definition.
(I've seen this before)
'greenest resorts with one' is the wordplay.
'resorts with' indicates an anagram.
'one' becomes 'i' (Roman numeral).
'greenest' mixed up with 'i' can make 'SERENGETI'.
'in' is the link.
(Other definitions for serengeti that I've seen before include "Afr. plain; green site (anag.)" , "where lion holds sway?" , "Tanzanian national park area" , "place to watch game" , "Tiger seen in Tanzania's great National Park" .)