In which case you're probably looking for an encyclopedia, rather than a dictionary.

The word you are looking for is 'patisserie'.

You'd go to an encyclopedia to find out the history, origins, detailed variations, regional specialities and so on about the topic of éclairs not to understand what an éclair itself is. A dictionary should perfectly well describe a thing without needing to pull out an encyclopedia and if you look at a updated/modern version of the Chambers Dictionary you'll get just such a definition: "A long cake of choux pastry with a cream filling and chocolate or coffee icing."