The demo in favour of, try to join (7)
I believe the answer is:
protest
'the demo' is the definition.
'in favour of try to join' is the wordplay.
'in favour of' becomes 'pro' (being pro a particular cause).
'try' becomes 'test' (test is a kind of try).
'to join' means one lot of letters go next to another.
'pro'+'test'='PROTEST'
(Other definitions for protest that I've seen before include "Disapprove" , "Record objection" , "Object loudly" , "Take exception" , "Statement or action showing disapproval" .)