Not sure that the part of speech in the answer quite matches the clue...I don't want to give the game away, and I'm not sure what I can say without doing that, but I would have clued it "Answer back" not "Answering back." (Sorry, I am as pedantic about crossword clues as I am about Maths)
Comments
Answering back at first, Rookie ECTs instructed listening. (6)