Small pale bird (4)
I believe the answer is:
swan
'bird' is the definition.
(I know that Swan is a type of bird)
'small pale' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'pale' becomes 'wan' (wan can mean pale or grey).
's'+'wan'='SWAN'
(Other definitions for swan that I've seen before include "Cob - pen" , "Large aquatic bird" , "The Ugly Duckling's outcome" , "black, in 7th" , "Stately aquatic bird" .)