Sap upset snake (3)
I believe the answer is:
asp
'snake' is the definition.
(asp is a kind of snake)
'sap upset' is the wordplay.
'upset' indicates an anagram.
'sap' anagrammed gives 'ASP'.
(Other definitions for asp that I've seen before include "Cleopatra-killing snake" , "A glider's what's called for" , "Cold-blooded killer" , "Small poisonous snake" , "Snake, killed Cleopatra" .)