Outdo previous top performer, by the sound of it (6)
I believe the answer is:
exceed
'outdo' is the definition.
(synonyms)
'previous top performer by the sound of it' is the wordplay.
'previous' becomes 'ex' (prefix meaning former or onetime).
'top performer' becomes 'seed' (eg tennis seeds).
'by the sound of it' indicates a 'sounds like' (homophone) clue.
'seed' is a homophone of 'ceed'.
'ex'+'ceed'='EXCEED'
(Other definitions for exceed that I've seen before include "Go past, outdo" , "Overshoot or be more than" , "Go beyond (a number)" , "Bud used as spice" , "Break (limit)" .)