Fear adder moving (5)
I believe the answer is:
dread
'fear' is the definition.
(I know that fear can be written as dread)
'adder moving' is the wordplay.
'moving' is an anagram indicator (letters are moved around).
'adder' with letters rearranged gives 'DREAD'.
(Other definitions for dread that I've seen before include "Show horror" , "Disquiet" , "Anticipate with horror" , "Anticipate fearfully" , "Enormous apprehension" .)