Deathly pale through seeing that bird (5)
I believe the answer is:
ashen
'deathly pale' is the definition.
(I've seen this before)
'seeing that bird' is the wordplay.
'seeing that' becomes 'as' (I've seen this in other clues).
'bird' becomes 'hen' (hen is a kind of bird).
'as'+'hen'='ASHEN'
'through' is the link.
(Other definitions for ashen that I've seen before include "Pale in complexion" , "Bloodless" , "showing signs of anaemia" , "Like bird" , "Very pale, with shock or fear" .)