Filled with bewilderment in the main (5)

I believe the answer is:

atsea

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'filled' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.

'with bewilderment in the main' is the wordplay.
'with' means one lot of letters go next to another.
'bewilderment' becomes 'at' (I can't explain this - if you can you should believe this answer much more).
'in the main' becomes 'sea' (the main is an old name for the sea).
'at' next to 'sea' is 'ATSEA'.

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I've seen this clue in The Times.

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