Swimmer's dull mind (4,5)

I believe the answer is:

grey nurse

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'swimmer's' is the definition.
I don't know anything about this answer so I can't tell whether it can be defined by this definition.

'dull mind' is the wordplay.
'dull' becomes 'grey' (I've seen this in another clue).
'mind' becomes 'nurse' (I can't explain this - if you can you should give a lot more credence to this answer).
'grey'+'nurse'='GREY NURSE'

Can you help me to learn more?

(Another definition for grey nurse that I've seen is " shark".)

I've seen this clue in the Sydney Morning Herald.

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