GMG made the decision after holding discussions with a number of agencies.
It has already revealed that it will rebrand its three Saga stations as Smooth stations, giving it a portfolio of five regional stations, which will grow to six this year.
Saga Radio won the licence to launch a new station in the North East before its sale to GMG. The station will launch in the autumn as a Smooth station.
Clear's main task is to create a new national brand for Smooth, but it will also work on revitalising the Real Radio and Century FM brands.
Geraldine Reilly, director of Clear, said: "Nothing is more satisfying than growing a client organically because it's testament to the fantastic work that we are already doing."