Snake going without sleep upset forest creature (3,5)
I believe the answer is:
red panda
'forest creature' is the definition.
(animal that lives in forests)
'snake going without sleep upset' is the wordplay.
'snake' becomes 'adder' (adder is a kind of snake).
'going without' indicates putting letters inside ('without' can mean 'outside').
'sleep' becomes 'nap' (napping is a kind of sleeping).
'upset' says the letters should be written backwards.
'adder' enclosing 'nap' is 'adnapder'.
'adnapder' reversed gives 'RED PANDA'.
(Other definitions for red panda that I've seen before include "Himalayan mammal" , "Wild animal" , "Raccoon-like mammal" , "Small carnivore - dared nap (anag)" , "something like a raccoon" .)