Secret arena newly built to accommodate a hundred (6)
I believe the answer is:
arcane
'secret' is the definition.
(thesaurus)
'arena newly built to accommodate a hundred' is the wordplay.
'newly built' indicates anagramming the letters.
'to accommodate' indicates putting letters inside.
'a hundred' becomes 'c' (roman numeral).
'arena' anagrammed gives 'arane'.
'arane' going around 'c' is 'ARCANE'.
(Other definitions for arcane that I've seen before include "Abstruse, mysterious" , "cryptic" , "Esoteric, mysterious" , "Requiring special knowledge" , "Mysterious, secret or obscure" .)