By finishing supper (3)

I believe the answer is:

per

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

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

'finishing supper' is the wordplay.
'PER' is hidden in amongst the letters.
However, I'm unsure how the hidden word is indicated.

Can you help me to learn more?

(Other definitions for per that I've seen before include "Preposition meaning for each" , "For each or every" , "Preposition indicating by means of" , "According to" .)

I've seen this clue in the Evening Standard.

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