Theatrical characters performing adult parts essentially (8)
I believe the answer is:
personae
'theatrical characters' is the definition.
'personae' can be an answer for 'characters' ('persona' can be a synonym of 'character'). I'm not sure about the 'theatrical' bit.
'performing adult parts essentially' is the wordplay.
'performing' becomes 'on' (as in being on stage).
'adult' becomes 'a' (abbreviation).
'parts' indicates putting letters inside (in the sense of cuts or separates).
'essentially' becomes 'per se' (I've seen this in another clue).
'on'+'a'='ona'
'ona' placed within 'perse' is 'PERSONAE'.
Can you help me to learn more?
(Other definitions for personae that I've seen before include "See 1" , "Theatrical roles" , "Dramatis ..., characters in a play" .)