Those who join in leave regularly in chic underwear (12)
I believe the answer is:
participants
'those who join in' is the definition.
(people who participate or join in)
'leave regularly in chic underwear' is the wordplay.
'leave' becomes 'part' (parting means leaving or separating).
'regularly in chic' becomes 'ici' (I can't explain this - if you can you should believe this answer much more).
'underwear' becomes 'pants' (I've seen this before).
'part'+'ici'+'pants'='PARTICIPANTS'
Can you help me to learn more?
(Another definition for participants that I've seen is " players".)