Bird that is small and pale (4)
I believe the answer is:
'bird' is the definition.
(swan is a kind of bird)
'small and pale' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'and' means one lot of letters go next to another.
'pale' becomes 'wan' (wan can mean pale or grey).
'that is' acts as a link.
(Other definitions for swan that I've seen before include "Long-necked water bird" , "Large, long-necked bird" , "Whooper or trumpeter?" , "Stately aquatic bird" , "black, in 7th" .)