Найди модуль StdCtrls, там этот компонент описан.
Код:
TComboBox = class(TCustomComboBox)
published
property AutoComplete default True;
property AutoDropDown default False;
property AutoCloseUp default False;
property BevelEdges;
property BevelInner;
property BevelKind default bkNone;
property BevelOuter;
property Style; {Must be published before Items}
property Anchors;
property BiDiMode;
property CharCase;
property Color;
property Constraints;
property Ctl3D;
property DragCursor;
property DragKind;
property DragMode;
property DropDownCount;
property Enabled;
property Font;
property ImeMode;
property ImeName;
property ItemHeight;
property ItemIndex default -1;
property MaxLength;
property ParentBiDiMode;
property ParentColor;
property ParentCtl3D;
property ParentFont;
property ParentShowHint;
property PopupMenu;
property ShowHint;
property Sorted;
property TabOrder;
property TabStop;
property Text;
property Visible;
property OnChange;
property OnClick;
property OnCloseUp;
property OnContextPopup;
property OnDblClick;
property OnDragDrop;
property OnDragOver;
property OnDrawItem;
property OnDropDown;
property OnEndDock;
property OnEndDrag;
property OnEnter;
property OnExit;
property OnKeyDown;
property OnKeyPress;
property OnKeyUp;
property OnMeasureItem;
property OnSelect;
property OnStartDock;
property OnStartDrag;
property Items; { Must be published after OnMeasureItem }
end;