Protest using a string of placard emojis (4)
I believe the answer is:
demo
'protest' is the definition.
(demonstration)
'a string of placard emojis' is the wordplay.
'a string of' indicates a hidden word (a string of letters from the clue).
'DEMO' is hidden in the letters of 'placard emojis'.
'using' is the link.
(Other definitions for demo that I've seen before include "Brass instrument valve" , "Show" , "presentation" , "Eg, protest march" , "Protest march (sl.)" .)