By John McCrankNEW YORK (Reuters) - A group of 24 investment management firms, including BlackRock Inc , Pimco and Eaton Vance , will begin clearing certain credit default swaps in an effort to revive a flagging part of the market used to hedge risk, several trade groups said on Wednesday.The effort comes amid renewed worries over the ability to trade debt following the collapse of Third Avenue's Focused Credit Fund on Dec. 7, which jolted Wall Street and sparked a sell-off in high-yield bonds. Low oil prices have also been putting pressure on some funds and companies.The investment management firms...