Translation:Say it clearly!
This sentence has proven very useful with the weird accents I have been hearing! Say it clearly!
This sentence actually implies that someone doesn't say something DIRECTLY. So açıkça literally means openly and it can mean frankly or directly depending on the context. If you want to tell someone to speak in a more understandable way you should use the word net (clear).
Açıkça söyle. - Say it openly. (directly, frankly)
Net (biçimde) söyle. - Say it clearly. (understandably, directly, frankly)
So I think net is the most appropriate and inclusive translation of 'clear'