Crossword Genius

Energy drink but not soft drink (5)

Ross

I believe the answer is:

pepsi

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

'soft drink' is the definition.
(I know that Pepsi is a soft drink)

'energy drink but not' is the wordplay.
I cannot really see how this works, but
'energy' could be 'pe' (P.E. is an example) and 'pe' is found in the answer.
The remaining letters 'psi' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...

Can you help me to learn more?

(Another definition for pepsi that I've seen is " drink".)

I've seen this clue in The Independent.
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