Used to be

WAS

Think of the past tense form of "is" that often appears before another verb, like in "It ___ raining yesterday.

The clue "Used to be" is a straightforward definition hinting at a past-tense verb. Here, "was" fits perfectly because it directly means something that existed or happened before. There’s no hidden wordplay—it’s a simple, direct reference to the verb’s basic meaning. Think of it like saying "I was there yesterday" to describe something in the past.