First to defeat champion swimmer (4)
I believe the answer is:
dace
'swimmer' is the definition.
(dace is a kind of fish)
'first to defeat champion' is the wordplay.
'first to' suggests taking the first letters.
'champion' becomes 'ace' (both can mean a person excelling at something).
The first letter of 'defeat' is 'd'.
'd'+'ace'='DACE'
(Other definitions for dace that I've seen before include "Fish in the carp family" , "Chubby swimmer" , "Freshwater fish" , "Small river fish" .)