Dread moving snake (5)
I believe the answer is:
adder
'snake' is the definition.
(I know that adder is a type of viper)
'dread moving' is the wordplay.
'moving' indicates anagramming the letters (letters are moved around).
'dread' is an anagram of 'ADDER'.
(Other definitions for adder that I've seen before include "He's making an extension" , "An arithmetic snake" , "Venemous snake" , "Type of snake" , "Viper common in northern Eurasia" .)