It's remarkable anger being voiced (5)
I believe the answer is:
great
'it's remarkable' is the definition.
(thesaurus)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for great that I've seen before include "Of considerable size" , "Important; immense" , "Of major significance - very good" , "Terrific" , "Very good, excellent" .)