Snake that may cause dread (5)
I believe the answer is:
adder
'snake' is the definition.
(adder is a kind of snake)
'that may cause dread' is the wordplay.
'that may cause' indicates an anagram (the same letters can create another word).
'dread' with letters rearranged gives 'ADDER'.
(Other definitions for adder that I've seen before include "danger in the countryside" , "venomous creature" , "exhibit in the reptile house" , "Britain's only venomous snake" , "Mathematical reptile" .)