'Underplay' in acting mean? How do you achieve it?

It's usually connected to a specific action or emotion. They want less emphasis on it. Whatever it is, the 'thing' needs to be there, but veiled and less obvious. Hard to explain without being in front of you physically, but I hope you get the jist.

Take a look at the difference between the way that David Suchet and Patrick Stewart approach Shylock. Here are some great examples of Suchet underplaying attributes of Shylock's character to very good effect.