Pale when on top of bird (5)
I believe the answer is:
ashen
'pale' is the definition.
('ashen' can be a synonym of 'pale')
'when on top of bird' is the wordplay.
'when' becomes 'as'.
'on top of' is a charade indicator (letters next to each other).
'bird' becomes 'hen' (hen is a kind of bird**).
'as'+'hen'='ASHEN'
(Other definitions for ashen that I've seen before include "Anaemic-looking" , "Pale-faced" , "may be pale with shock" , "Anaemic looking" , "pasty" .)