Halliday & Matthiessen (2014: 495-6):
Note that in examples such as the first person who came in, the best person to do the job, the embedded clause strictly has as its domain not the Head noun person but a premodifying element; the meaning is ‘the first-who-came-in person’, ‘the best-to-do-the-job person’. Compare a hard act to follow, the longest bridge ever built. We can express this relationship structurally as in Figure 7-16.
But as already pointed out, constituency is not a very appropriate concept for representing semantic domain, and for most purposes it suffices to show the clause simply as embedded in the nominal group: a hard act [[to follow]].