Denizens of forest run out of trousers (7)

I believe the answer is:

beeches

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

This could be a double definition.

'denizens of forest run' is the first definition.
Although both the answer and definition are plural nouns, I cannot understand how one could define the other.

'trousers' is the second definition.
Although both the answer and definition are plural nouns, I can't understand how one could define the other.

'out of' is the link.

Can you help me to learn more?

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.