Gun down a wildebeest (3)

I believe the answer is:

gnu

I'm a little stuck... Click here to teach me more about this clue!

'wildebeest' is the definition.
(I know that wildebeest can be written as gnu)

'gun down a' is the wordplay.
'gun' anagrams to 'GNU'.
But, I'm not sure how this is indicated.

Can you help me to learn more?

(Other definitions for gnu that I've seen before include "animals" , "Gun to get antelope" , "Creature" , "hitherto unknown, we hear" , "Animal" .)

I've seen this clue in The Irish Independent.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.