Explore underground pool, with most of them swimming (7)

I believe the answer is:

pothole

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'explore underground' is the definition.
I can't tell whether this definition defines the answer.

'pool with most of them swimming' is the wordplay.
'with' says to put letters next to each other.
'most of' means to remove the last letter.
'swimming' indicates anagramming the letters.
'them' with its final letter removed is 'the'.
'pool'+'the'='poolthe'
'poolthe' anagrammed gives 'POTHOLE'.

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(Other definitions for pothole that I've seen before include "Small pit in road surface" , "that can be problem when driving" , "Damage to road" , "Water-eroded cave" , "Deep cave; road hazard" .)

I've seen this clue in The Telegraph.

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