Complex sentence: At the request of Clarke's agent, and unknown to Clarke, the story was edited by James Blish and given a new ending, after which it was accepted for publication in April 1950 by "Famous Fantastic Mysteries" magazine.
Split into simpler sentences: At the request of Clarke's agent (and unbeknownst to Clarke) the story was edited by James Blish who rewrote the ending. Blish's version of the story was accepted for publication in April 1950 by "Famous Fantastic Mysteries" magazine.