He deals with the customers in a very 'professional' way. = He is very 'professional' in dealing with the customers.
a golf/tennis 'professional' [=a skillful golf/tennis player whose job is to teach other people how to play]
I was impressed by the calm and 'professional' way she handled the crisis.
(..) You better shit-can that blasphemy, boy. You're in a house of worship. Sorry, Pop. Well, this is definitely the work of professionals. I'd guesstimate Mexican Mafia hit squad. Four, maybe five strong. How can you tell? Well, a sure and steady hand (..)2003 Kill Bill: Vol. 1