Energies in stomach enzyme (6)

I believe the answer is:

pepsin

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'stomach enzyme' is the definition.
(I've seen this in another clue)

'energies in' is the wordplay.
'energies' becomes 'peps' (I can't justify this - if you can you should believe this answer much more).
'peps'+'in'='PEPSIN'

Can you help me to learn more?

(Another definition for pepsin that I've seen is " enzyme".)

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

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