Small, pale-coloured bird (4)
I believe the answer is:
swan
'bird' is the definition.
(I know that Swan is a type of bird)
'small pale-coloured' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'pale-coloured' becomes 'wan' (wan means pale or ashen).
's'+'wan'='SWAN'
(Other definitions for swan that I've seen before include "possibly mute" , "Kind of song, in the end" , "The Ugly Duckling's outcome" , "Shakespeare is called The .... of Avon" , "Mute, whooper or Bewick's bird" .)