I'm in a thrash band. I play lead/rhythm guitar and sing. My guitar player has told me to try to more in a clean voice like anthrax, but still sound real agressive. I'm just wondering how to go about that. Any serious advice is appreciated.
Well, there's a good chance that your whole singing career has been based on this more aggressive type of singing. My only advice would be to sing some cleaner thrash songs (Such as Anthrax) while you are at home and before practice. Doing this should help you develop your clean voice.

Remember practice makes perfect.