Swimming tutor, possibly? (5)

I believe the answer is:

trout

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'swimming' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.

'tutor possibly?' is the wordplay.
'possibly?' indicates an anagram (I've seen 'possibly' mean this).
'tutor' is an anagram of 'TROUT'.

Can you help me to learn more?

(Other definitions for trout that I've seen before include "Tutor (anag.)" , "Commonly caught fish" , "one of a noted quintet" , "Unattractive old woman" , "Catch" .)

I've seen this clue in the Evening Standard.

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