The porpoise's tail is for balance (5)

I believe the answer is:

poise

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

'is for balance' is the definition.
(I've seen this before)

'the porpoise's tail' is the wordplay.
'POISE' is hidden within the letters.
But, I am unsure how this is indicated.

Can you help me to learn more?

(Other definitions for poise that I've seen before include "cool" , "Dignity and assurance of manner" , "Balance; composure" , "Aplomb" , "Elegance" .)

I've seen this clue in the Evening Standard.

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