Someone or something so exemplary that it overshadows anything that follows. I don't know how Mel's replacement will be received by the staff. He was so admired and respected that he'll be a tough act to follow.That award-winning play will be a tough act to follow on Broadway.
Fig. a difficult presentation or performance to follow or improve upon with one's own performance. Bill's speech was excellent. It was a tough act to follow, but my speech was good also.In spite of the fact that I had a tough act to follow, I did my best.