Viper (5)
I believe the answer is:
adder
'viper' is the definition.
(adder is a kind of viper)
This is the entire clue.
(Other definitions for adder that I've seen before include "a poisonous type" , "venomous creature" , "Totting snake" , "Viper common in northern Eurasia" , "Snake (like an abacus?)" .)