Flier, best swimmer (8)
I believe the answer is:
starling
'flier' is the definition.
(starling is a kind of birdanimal)
'best swimmer' is the wordplay.
'best' becomes 'star' (star can mean the best or most excellent).
'swimmer' becomes 'ling' (ling is a type of fish).
'star'+'ling'='STARLING'
(Other definitions for starling that I've seen before include "Slant rig for the strange bird" , "Passerine bird" , "Bird from heavenly body and fish" , "is naturally a good mimic" , "Common songbird" .)