Adult in party lay bare (6)
I believe the answer is:
reveal
'lay bare' is the definition.
(thesaurus)
'adult in party' is the wordplay.
'adult' becomes 'a' (abbreviation).
'in' means one lot of letters goes inside another.
'party' becomes 'revel' (I've seen this before).
'a' put within 'revel' is 'REVEAL'.
(Other definitions for reveal that I've seen before include "Remove cover or expose to view" , "Final disclosure" , "Remove cover to display" , "Make disclosure" , "window, in part" .)