Be very afraid of adder (5)
I believe the answer is:
dread
'be very afraid' is the definition.
(I've seen this in another clue)
'of adder' is the wordplay.
'of' indicates anagramming the letters (the answer comes from or 'of' the letters).
'adder' anagrammed gives 'DREAD'.
(Other definitions for dread that I've seen before include "Terror" , "Trepidation" , "Deep fear" , "Anticipate with fear" , "Hairstyle" .)