Surgery that's good for swimmer (4)

I believe the answer is:

opah

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'swimmer' is the definition.
(opah is a kind of fish)

'surgery that's good' is the wordplay.
'surgery' becomes 'op' (short for operation).
'that's good' becomes 'ah' (I can't justify this - if you can you should believe this answer much more).
'op'+'ah'='OPAH'

'for' is the link.

Can you help me to learn more?

(Other definitions for opah that I've seen before include "Large deep-sea fish" , "Ocean swimmer" , "Kingfish" .)

I've seen this clue in The Independent.

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