Small, pale swimmer (4)
I believe the answer is:
swan
'swimmer' is the definition.
(swans can swim)
'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 "black, in 7th" , "Royal bird" , "Cob - pen" , "possibly mute" , "Bird of Cygnus genus" .)