Number one seed's cut in half racket (5)
I believe the answer is:
noise
'racket' is the definition.
(racket can mean a loud noise)
'number one seed's cut in half' is the wordplay.
'number' becomes 'no' (abbreviation).
'one' becomes 'i' (Roman numeral).
'cut in half' suggests halving the letters.
'seed' with half the letters taken is 'se'.
'no'+'i'+'se'='NOISE'
(Other definitions for noise that I've seen before include "Clamour" , "Virgil's birthplace" , "Public talk" , "Unwanted sound" , "Unwanted sounds" .)