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.
(..) Sports too. You gotta move, you know. For the social life, I mean. These people can't do anything... I saw... had my first professional experience... It was Mme Dupont-Moretti. A really old lady... really old... I assisted her... until the end, day after day. (..)2011 The Intouchables