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.
(..) Machine-Man from a mortal--!" "The woman is m i n e, Joh Fredersen! The son of Hel was yours!" "And what brings you to me, Joh Fredersen?" "As usual, when my experts fail me, I come to you for advice..." "For months now, we have been finding these plans in my workers' clothes. What do they mean?" (..)1927 Metropolis