A person who cannot let anything go by. They always have to answer back in a condesending way. These are usually people who are higher in the academic world than their peers and they use their knowledge in a mean way.
"He had to say that just to be a clever trousers. Who cares if pigs don't really sweat that much?"
Someone who makes snide remarks, or otherwise is acting in a fashion that makes others believe they are being smug.
British in origin.
Exactly the same as Smarty Pants.
"Oh, you're such a clever trousers with your puns, AREN'T you, Neil?"