By word of mouth, round racing event (6)
I believe the answer is:
orally
'by word of mouth' is the definition.
(I've seen this before)
'round racing event' is the wordplay.
'round' becomes 'o' (o is a circular letter).
'racing event' becomes 'rally'.
'o'+'rally'='ORALLY'
(Other definitions for orally that I've seen before include "How anyone would speak" , "given drugs thus?" , "By spoken word" , "How one speaks" , "via the trap" .)