Crossword Genius

They serve as predicates in sentences (5)

Ross

I believe the answer is:

verbs

(Another definition for verbs that I've seen is " Parts of speech".)

I've seen this clue in The Irish Times.
Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.