Food waiter keeps fifty and discards one with the excess fluid (10)

I believe the answer is:

floodwater

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'discards one with the excess fluid' is the definition.
I can't tell whether this definition defines the answer.

'food waiter keeps fifty' is the wordplay.
'waiter' becomes 'water' (I can't justify this - if you can you should believe this answer much more).
'keeps' is an insertion indicator (inserted letters are kept inside).
'fifty' becomes 'l' (Roman numeral).
'food'+'water'='foodwater'
'foodwater' enclosing 'l' is 'FLOODWATER'.

'and' acts as a link.

Can you help me to learn more?

(Other definitions for floodwater that I've seen before include "A devastating rise" , "Cause of inundation" .)

I've seen this clue in the Sydney Morning Herald.

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