Dread abnormal snake (5)
I believe the answer is:
adder
'snake' is the definition.
(I know that adder is a type of viper)
'dread abnormal' is the wordplay.
'abnormal' indicates anagramming the letters.
'dread' anagrammed gives 'ADDER'.
(Other definitions for adder that I've seen before include "exhibit in the reptile house" , "Venemous snake" , "Viper common in northern Eurasia" , "a poisonous type" , "Venomous type" .)