Top seed's English racket (5)
I believe the answer is:
noise
'racket' is the definition.
(synonyms)
'top seed's english' is the wordplay.
'top seed's' becomes 'No I's' (resembles No. 1, top seeded player).
'english' becomes 'E' (abbreviation).
'nois'+'e'='NOISE'
(Other definitions for noise that I've seen before include "Clatter" , "Kind of pollution" , "Loud or disturbing sound" , "Unwanted sound" , "Row" .)