The more they cut down the more they gain (10)
I believe the answer is:
Here is my best explanation:
'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 can't justify this - if you can you should believe this answer much more).