A gun could get it beastly upset! (3)
I believe the answer is:
gnu
'it beastly upset' is the definition.
The definition suggests a singular noun which matches the answer.
'a gun could' is the wordplay.
'gun' anagrams to 'GNU'.
However, I'm unsure how the anagram is indicated.
'get' acts as a link.
Can you help me to learn more?
(Other definitions for gnu that I've seen before include "Wildebeest" , "Strange gun for an antelope" , "animals" , "Animal" , "Antelope sounds as if it has knowledge" .)