Dog entered in races is withdrawn (9)
I believe the answer is:
introvert
'is withdrawn' is the definition.
(I've seen this before)
'dog entered in races' is the wordplay.
'dog' becomes 'rover' (I've seen this before).
'entered' means one lot of letters goes inside another.
'races' becomes 'tt' (Isle of Man Tourist Trophy races).
'in'+'tt'='intt'
'rover' put into 'intt' is 'INTROVERT'.
(Other definitions for introvert that I've seen before include "eschewing external contact" , "Rover tint (anag.)" , "Shy type" , "Self-absorbed person" , "One who's withdrawn" .)