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.
(..) What's the trouble? This man says we have trouble with our telephones. Allow me to introduce myself. I am Emile Flournoy, communications expert and chief troubleshooter for the Nice telephone company. Oh, please come in, monsieur. Now what can we do to help you? (..)1975 The Return of the Pink Panther