Savanna grazer (3)
(Other definitions for gnu that I've seen before include "Strange gun for an antelope" , "Animal" , "Literally wild beast" , "Gun for a kind of deer" , "Gun to get antelope" .)
'savanna grazer' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for impala that I've seen before include "Graceful antelope" , "leaping creature" , "Spectacular jumper" , "One regularly browsing" , "Breed of antelope" .)