Be afraid of disturbing adder (5)
I believe the answer is:
dread
'be afraid of' is the definition.
(I've seen this before)
'disturbing adder' is the wordplay.
'disturbing' is an anagram indicator (I've seen this in other clues).
'adder' anagrammed gives 'DREAD'.
(Other definitions for dread that I've seen before include "Be scared of" , "Frightful" , "alarm" , "Adder (anag)" , "Get cold feet" .)