Everyone in extremely rowdy mass meeting (5)
I believe the answer is:
rally
'mass meeting' is the definition.
('rally' can be a synonym of 'mass meeting')
'everyone in extremely rowdy' is the wordplay.
'everyone' becomes 'all' (I've seen this before).
'in' is an insertion indicator.
'extremely' means to remove the middle letters.
'rowdy' with its centre removed is 'ry'.
'all' inserted into 'ry' is 'RALLY'.
(Other definitions for rally that I've seen before include "March" , "Recover; car race" , "Motor sport event" , "Muster" , "Recover health" .)