They rise or fall to some degree (12)

I believe the answer is:

thermometers

I'm a little stuck... Click here to teach me more about this clue!

'they' is the definition.
Both the answer and definition are plural nouns.
Perhaps they are linked in a way I don't understand?

I don't understand the remainder of the clue.

Can you help me to learn more?

I've seen this clue in The Globe and Mail.

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.