The first sign of something ending comes from:
The first sign of something [that is] ending.
It is a reduced relative clause.
reduced relative clause
And here is one rule for them from the "Grammar Bank" site:
- If "To be" verb is used after a relative pronoun we can omit "Relative Pronoun + To be".
The car which is parked next to mine is very expensive.
The car parked next to mine is very expensive.
Hamlet, which was written by Shakespeare sometime in the early 1600s, is among the classics.
Hamlet, written by Shakespeare sometime in the early 1600s, is among the classics.
remove that + to be
All the sites I can find gives examples of clauses like this, where the auxiliary verbs can be more than a single word verb. The auxiliaries such as: that is [verb], that are [verb] are all shown as examples of reduced clauses.