It might have been in one of sho_no_tabi's fics, since her Arthur is always Brendan!Arthur and he has occasionally worn a pinstripe suit & fedora. I think I vaguely remember an instance when he wanted to play up the mafia image, but I can't remember which fic it was.

If you haven't read her fics, you MUST. SHE IS SO FUCKING ABSOLUTELY AMAZING. But go to her masterlist and start with A Soul Awake one because they're all in the same verse, and they're published in chronological order.