Sore loser's first returning racket (6)
I believe the answer is:
lesion
'sore' is the definition.
(thesaurus)
'loser's first returning racket' is the wordplay.
'first' suggests taking the first letters.
'returning' says the letters should be written backwards.
'racket' becomes 'noise' ('noise' can be a synonym of 'racket'**).
The first letter of 'losers' is 'l'.
'noise' back-to-front is 'esion'.
'l'+'esion'='LESION'
(Other definitions for lesion that I've seen before include "Change in bodily tissue form injury or disease" , "Kind of injury in insole" , "A cut or wound in insole" , "Abrasion" , "Flesh wound" .)