Attend a protest (3,2)
I believe the answer is:
sit in
This clue is a double definition.
'attend' is the first definition.
(thesaurus)
'a protest' is the second definition.
(sit-in is a type of non-violent protest)
(Other definitions for sit in that I've seen before include "Act as a substitute for" , "demonstration" , "form of protest" , "act as someone's deputy" .)