I was given a copy of Chambers Dictionary[0] by my sister for my 18th birthday, I still have it. According to wiki, it is "widely used by British crossword solvers and setters, and by Scrabble players[..] It contains many more dialectal, archaic, unconventional and eccentric words than its rivals, and is noted for its occasional wryly humorous definitions".
Wow! That's certainly a good one. A couple old favorites of mine are:
mullet – a hairstyle that is short at the front, long at the back, and ridiculous all around, and
regift – to give (an unwanted present) as a gift to another person, in a process which is likely to continue almost indefinitely.
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."
I've always loved its definition of "éclair":
> "a cake, long in shape but short in duration"
[0] https://en.wikipedia.org/wiki/Chambers_Dictionary