Swimmers used to swim in West? (7)

I believe the answer is:

medusae

'swimmers' is the definition.
(plural of medusa, type of jellyfish)

'used to swim in west?' is the wordplay.
'to swim' indicates anagramming the letters.
'in' indicates putting letters inside.
'west?' becomes 'mae' (I've seen this before).
'used' anagrammed gives 'edus'.
'edus' placed into 'mae' is 'MEDUSAE'.

I've seen this clue in The Times.

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