On to enter an adult scene (5)
I believe the answer is:
arena
'scene' is the definition.
(I've seen this before)
'on to enter an adult' is the wordplay.
'on' becomes 're' ('on' can be mean 'with reference to').
'to enter' is an insertion indicator.
'adult' becomes 'a' (abbreviation).
're' going within 'an' is 'aren'.
'aren'+'a'='ARENA'
(Other definitions for arena that I've seen before include "Space for public events" , "Sphere of intense activity" , "Place of combat" , "Show ring" , "domain" .)