Champion with knight's bird (5)
I believe the answer is:
heron
'bird' is the definition.
(I know that heron is a bird)
'champion with knight's' is the wordplay.
'champion' becomes 'hero' (synonyms).
'with' says to put letters next to each other.
'knight' becomes 'N' (chess abbreviation).
'hero'+'n'='HERON'
(Other definitions for heron that I've seen before include "Big bird" , "One bird" , "Long-legged long-necked wading bird" , "Large fish-eating bird" , "Bird of the Rhone, strangely" .)