an 'expert witness' [=a witness in a court of law who is an expert on a particular subject]
The company has become 'expert' at/in adapting its products for new clients.
She was an acknowledged 'expert' on/in child development.
(..) The love experts? Love experts?! And don't worry, they'll be able to fix this. How do you know? Because I've seen them do it before. I like to consider myself a love expert. Get it together. Control it. Don't feel. Don't feel. Don't feel! Look, Sven. The sky's awake. Are you cold? A little. (..)2013 Frozen
(..) Excuse me, Sir. He's a Prince. All men do it. Ew. Look it doesn't matter, it's true love. Doesn't sound like true love. Are you some sort of love expert? No. But I have friends who are. You have friends who are love experts? I'm not buying it...Stop talking. No, no, no. I'd like to meet these... (..)2013 Frozen