They say the fourth witch dumped Charlie immediately (9)

I believe the answer is:

forthwith

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'immediately' is the definition.
(I know that immediately can be written as forthwith)

'they say the fourth witch' is the wordplay.
'they say' shows a homophone (sound like).
'witch' becomes 'with' (I am not sure about this - if you are sure you should believe this answer much more).
'fourth' sounds like 'forth'.
'forth'+'with'='FORTHWITH'

'dumped charlie' is the link.
This may not be correct. Some or all of it may belong to another bit of the clue.

Can you help me to learn more?

(Other definitions for forthwith that I've seen before include "without further delay - at which it starts, audibly" , "Without delay or hesitation" , "on the spot" , "Immediately as with froth" .)

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

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