One taking part pains the jockeys (8)
I believe the answer is:
thespian
'one taking part' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'pains the jockeys' is the wordplay.
'jockeys' is an anagram indicator.
'pains'+'the'='painsthe'
'painsthe' with letters rearranged gives 'THESPIAN'.
Can you help me to learn more?
(Other definitions for thespian that I've seen before include "Tragic actor" , "Dramatic actor" , "Theatrical performer" , "He paints (anag)" , "Actor; relating to drama" .)