Snake upset king (7)
I believe the answer is:
rattler
'snake' is the definition.
(I know that rattler is a type of pit viper)
'upset king' is the wordplay.
'upset' becomes 'rattle' (I've seen this before).
'king' becomes 'R' (abbreviation for rex, king in Latin).
'rattle'+'r'='RATTLER'
(Other definitions for rattler that I've seen before include "Nickname for a pit viper in the US" , "(it crawls?)" , "old crate" , "American pit viper (informal)" , "A noisy American snake" .)