Definition: "he's a trier!"

This uses letter substitution - one letter from the NATO phonetic alphabet is replaced with another letter from the NATO alphabet.

Start with 'Barnaby' - this refers to the Dickens character Barnaby Rudge. Remove the letter R (Romeo in NATO alphabet) and replace it with J (Juliet in NATO alphabet).

Answer: JUDGE | Take Barnaby RUDGE (Dickens character), remove R (Romeo) and replace with J (Juliet) to get JUDGE. A judge is 'a trier' because they try court cases.