The bottom ones earn her the greatest income (10)
I believe the answer is:
nethermost
'the bottom' is the definition.
(I've seen this before)
'earn her the greatest income' is the wordplay.
'earn' becomes 'net' (I've seen this in another clue).
'the greatest income' becomes 'most' (I can't justify this - if you can you should give a lot more credence to this answer).
'net'+'her'+'most'='NETHERMOST'
'ones' is the link.
This may not be correct. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for nethermost that I've seen before include "located in lowest position" , "at the bottom of the pile" .)