Immature nude naughtily joining rave (8)
I believe the answer is:
underage
'immature' is the definition.
(I've seen this before)
'nude naughtily joining rave' is the wordplay.
'naughtily' is an anagram indicator.
'joining' is a charade indicator (letters next to each other).
'rave' becomes 'rage' (both can mean to behave angrily).
'nude' with letters rearranged gives 'unde'.
'unde'+'rage'='UNDERAGE'
(Other definitions for underage that I've seen before include "Too tender" , "Not yet eligible to vote - too young" , "one too young" , "Not old enough to exercise legal right" , "Need a rug (anag.)" .)