One sport he worked on, nearing defeat (2,3,5)
I believe the answer is:
on the ropes
'nearing defeat' is the definition.
'one sport he worked' is the wordplay.
'worked' is an anagram indicator.
'one'+'sport'+'he'='onesporthe'
'onesporthe' anagrammed gives 'ON THE ROPES'.
'on' is the link.
(Other definitions for on the ropes that I've seen before include "Close to collapse" , "close to defeat" , "In a hopeless position" , "In a state of near collapse" , "Badly placed in the ring" .)