Plumbers may be all at sea when taking them (9)

I believe the answer is:

soundings

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'sea when taking them' is the definition.
I know nothing about this answer so I cannot judge whether this works.

'plumbers may be all at' is the wordplay.
I cannot really understand how this works, but
'at' could be 'in' (synonymous in some cases - eg at school, in school) and 'in' is present in the answer.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?

I've seen this clue in The Irish News.

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