Plain English Meaning
Hides extra stuff on the screen so you can see what matters right now.
Full Definition
Automatically removes non-essential symbols, data, and information from a cockpit display so that only the information needed for the current task or flight situation remains visible.
Context Anchor
Seen in descriptions of electronic flight displays and unusual attitude recovery protection, where the screen may simplify itself during recovery.
Derivation
From 'clutter' (a disordered heap or mess) with the prefix 'de-' meaning 'remove' or 'undo.' To de-clutter is literally to take the mess away. The aviation use applies this directly to a screen full of symbols.