Crossword Genius

Swim up and down with pronounced bad posture (8)

Ross

I believe the answer is:

porpoise

I'm a little stuck... Click here to teach me more about this clue!

'down with pronounced bad posture' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.

'swim up' is the wordplay.
I cannot really see how this works, but
'up' could be 'is' (I've seen this in other clues) and 'is' is found in the answer.
This explanation may well be incorrect...

'and' acts as a link.

Can you help me to learn more?

(Other definitions for porpoise that I've seen before include "Type of dolphin" , "Animal" , "Small aquatic mammal with blunt snout" , "Swimmer" , "Dolphin-like beast" .)

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