Dog's entered in races as one withdrawn (9)
I believe the answer is:
introvert
'one withdrawn' is the definition.
(I've seen this before)
'dog's entered in races' is the wordplay.
'dog' becomes 'rover' (I've seen this before).
'entered' indicates putting letters inside.
'races' becomes 'tt' (Isle of Man Tourist Trophy races).
'in'+'tt'='intt'
'rover' put into 'intt' is 'INTROVERT'.
'as' acts as a link.
(Other definitions for introvert that I've seen before include "Turn inwards" , "quiet" , "One who's withdrawn" , "one's reserved" , "eschewing external contact" .)