African plain's surprisingly greenest before onset of inundation (9)
I believe the answer is:
serengeti
'african plain's' is the definition.
(geographic region in Tanzania)
'surprisingly greenest before onset of inundation' is the wordplay.
'surprisingly' is an anagram indicator (a surprising ordering of the letters).
'before' is a charade indicator (letters next to each other).
'onset of' suggests taking the first letters.
The initial letter of 'inundation' is 'i'.
'greenest' with letters rearranged gives 'serenget'.
'serenget'+'i'='SERENGETI'
(Other definitions for serengeti that I've seen before include "region in Tanzania" , "where lion holds sway?" , "Tanzanian national park area" , "African ecosystem" , "Tiger seen in Tanzania's great National Park" .)