Adult near Barking Stadium (5)
I believe the answer is:
'stadium' is the definition.
(I know that stadium can be written as arena)
'adult near barking' is the wordplay.
'adult' becomes 'a' (abbreviation).
'barking' is an anagram indicator (UK slang for mad).
'near' is an anagram of 'rena'.
(Other definitions for arena that I've seen before include "Area of combat" , "Place of public contest" , "Contest venue" , "Place of great activity" , "Area in centre of sports stadium" .)