Unknown actor Johnny, acting up, kept on talking (6)
I believe the answer is:
yapped
'unknown' is the definition.
The answer and definition are not the same part of speech. However, adjectives and past participle verbs can sometimes mean the same thing.
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for yapped that I've seen before include "Barked like a small dog" , "Yelped" , "Chatted" .)