Fear movement of adder (5)
I believe the answer is:
dread
'fear' is the definition.
(synonyms)
'movement of adder' is the wordplay.
'movement of' indicates an anagram (I've seen 'movement' mean this (letters move around)).
'adder' anagrammed gives 'DREAD'.
(Other definitions for dread that I've seen before include "Adder (anag)" , "Frightful" , "alarm" , "Be afraid of - anxiety" , "Trepidation" .)