So you made your cut and the piece is still just a hair too long. All you need is the tiniest trim off this piece and it will fit perfectly, but sometimes taking off just a little is difficult to set up.
To just shave the piece set it up against the blade (with the saw off!), then press a little harder against the blade until you see the blade bend just little, hold the piece tightly against the miter gauge and slide it back well clear of the blade. Turn the saw on, and make your cut. You will take off just a hair.