Free from impurities, eels can swim (7)

I believe the answer is:

cleanse

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'free from' is the definition.
(thesaurus)

'eels can swim' is the wordplay.
'swim' indicates anagramming the letters.
'eels'+'can'='eelscan'
'eelscan' is an anagram of 'CLEANSE'.

'impurities' acts as a link.
I am not very happy about this link. It may be part of another bit of the clue.

Can you help me to learn more?

(Other definitions for cleanse that I've seen before include "do some laundering?" , "Make thoroughly clean" , "Make free from dirt" , "Make pure" , "Scrub" .)

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