Definition
The aerodynamic state of an airplane in flight with the landing gear retracted (if retractable) and the flaps fully retracted, so that no high-drag devices are extended from the wings or fuselage.
Plain English
The airplane is set up with everything tucked in -- wheels up (if they retract) and flaps up -- so the wings and body are smooth with nothing sticking out into the airflow.
Context Anchor
Seen in slow flight, stall, takeoff, climb, cruise, and performance discussions when the handbook describes how the airplane is set up for the maneuver or condition.
Derivation
"Clean" here means uncluttered -- nothing protruding into the airstream. The wing's surface is left in its smoothest, lowest-drag shape, the way it was designed to fly in cruise.
Why Pilots Care
A clean configuration gives the lowest stall speed for a given weight and allows precise control of airspeed with minimal power changes.
Intuition Check
Clean does not mean washed, polished, or undamaged here. It means the airplane has its normal drag-adding devices, such as flaps and retractable landing gear, retracted.
Example Sentence 1
The instructor asked the student to enter slow flight in the clean configuration before adding flaps.
Example Sentence 2
Once established in the traffic pattern the pilot transitioned from the clean configuration by lowering the flaps and gear.