Plain English Meaning
Looking around carefully before you start a maneuver to make sure no other aircraft are in your way or about to be.
Full Definition
A visual scan of the surrounding sky performed before initiating a maneuver, used to ensure no other aircraft are in a position that could create a conflict during the maneuver. Typically accomplished with clearing turns or a series of gentle banks that allow the pilot to see into areas otherwise blocked by the aircraft structure.
Context Anchor
Used during flight training before an instructor or student demonstrates or practices a maneuver.
Derivation
Clear comes from an older word meaning bright or unobstructed. In aviation, the idea is not brightness; it is that the space is unobstructed and safe to use.