One who believes he is taking part in race (6)
I believe the answer is:
theist
'one who believes' is the definition.
(I've seen this before)
'he is taking part in race' is the wordplay.
'taking part in' is an insertion indicator.
'race' becomes 'tt'.
'he'+'is'='heis'
'heis' going inside 'tt' is 'THEIST'.
(Other definitions for theist that I've seen before include "One believes in the existence of a god or gods" , "Believer in god/gods" , "Religious type" , "One who believe in a god" , "Believer in a god or gods" .)