Act of coming in again (7)
(Other definitions for re-entry that I've seen before include "Return from outer space" , "Spacecraft's return phase" , "Hitting earth's atmosphere again" , "Spacecraft's return to earth" , "A spacecraft's track coming back into atmosphere" .)
(Other definitions for reentry that I've seen before include "final part of space flight" , "Return to the Earth's atmosphere" , "of concern to NASA?" .)