Am all upset by a backward beast (5)
I believe the answer is:
llama
'a backward beast' is the definition.
'llama' can be an answer for 'beast' (I've seen this before). I am not certain of the 'a backward' bit.
'am all upset' is the wordplay.
'upset' says the letters should be written in reverse.
'am'+'all'='amall'
'amall' backwards is 'LLAMA'.
'by' is the link.
Can you help me to learn more?
(Other definitions for llama that I've seen before include "Andean camelid" , "South American relative of the camel" , "Camel-like wool source" , "one that ruminates" , "South American native" .)