One part of bird thats causing dread (5)
I believe the answer is:
awing
'causing dread' is the definition.
(I've seen this before)
'one part of bird' is the wordplay.
'one' becomes 'a'.
'part of bird' becomes 'wing' ('wing' is part of 'bird').
'a'+'wing'='AWING'
'thats' is the link.
(Other definitions for awing that I've seen before include "Creating reverence" , "inspiring with dread" , "Causing dread" , "making an impression" , "inspiring great admiration" .)