Tempers are initially absent where unmarried couples live together? (5,2)

I believe the answer is:

reins in

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

'tempers' is the definition.
Although both the answer and definition are verbs in their -s form, I don't understand how they can define each other.

I can't explain the rest of the clue.

Can you help me to learn more?

(Another definition for reins in that I've seen is " Checks".)

I've seen this clue in The Guardian.

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.