Person training in the arena regularly (6)
I believe the answer is:
intern
'person training' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'in the arena regularly' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'the'+'arena'='thearena'
The alternate letters of 'thearena' are 'tern'.
'in'+'tern'='INTERN'
Can you help me to learn more?
(Other definitions for intern that I've seen before include "Student" , "Confine as a prisoner" , "to keep under lock and key" , "Work-experience trainee" , "Young American 18" .)