One who believes he is taking part in race (6)
I believe the answer is:
theist
'one who believes' is the definition.
(someone who believes in a god)
'he is taking part in race' is the wordplay.
'taking part in' is an insertion indicator.
'race' becomes 'tt'.
'he'+'is'='heis'
'heis' inserted into '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" .)