Crossword Genius

For each piece of wallpaper (3)

Ross

I believe the answer is:

per

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

'for each' is the definition.
('per' can mean for each)

'piece of wallpaper' is the wordplay.
'PER' is hidden in amongst the letters.
However, I am uncertain how this is indicated.

Can you help me to learn more?

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

I've seen this clue in the Evening Standard.
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