Knows beyond doubt there's plenty of water (4,4)

'knows beyond doubt' is the definition.
I don't know anything about this answer so I can't tell whether it can be defined by this definition.

'there's plenty of water' is the wordplay.
'there's' is a charade indicator (letters next to each other).
'plenty of' becomes 'full' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'water' becomes 'well water' (well water is a kind of water).
'full' put next to 'well' is 'FULL WELL'.

Can you help me to learn more?