The more they cut down, the more they gain (10)

I believe the answer is:

harvesters

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

'the more they' is the definition.
The definition suggests a plural noun which matches the answer.

'cut down the more they gain' is the wordplay.
'cut down' becomes 'harvest' (I've seen this before).
'the more they gain' becomes 'ers' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'harvest'+'ers'='HARVESTERS'

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.