Green’s Dictionary of Slang

cheap claret; also attrib.

1864, 1867, 1870		Hotten Sl. Dict.
1876		Besant & Rice Golden Butterfly I 170: Claret certainly good, too – none of your Gladstone tap; sherry probably rather coarse.
1885		A. Birrell Obiter Dicta 86: To make him unbosom himself over a bottle of Gladstone claret in a tavern in Leicester Square [F&H].