Be afraid of disturbing an adder (5)
I believe the answer is:
dread
'be afraid of' is the definition.
(I've seen this in another clue)
'disturbing an adder' is the wordplay.
'disturbing' indicates an anagram (I've seen this in other clues).
'adder' is an anagram of 'DREAD'.
(Other definitions for dread that I've seen before include "Great fear or apprehension" , "great anxiety" , "Dared to fear greatly" , "Adder (anag)" , "alarm" .)