Crossword Genius

One who betrays an interest in buying? (7)

Ross

I believe the answer is:

shopper

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

'one who betrays' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.

'an interest in buying?' is the wordplay.
I cannot really understand how this works, but
'an' could be 'per' (synonymous as in 'once an hour', 'once per hour') and 'per' is found in the answer.
The remaining letters 'shop' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for shopper that I've seen before include "basket" , "selling?" , "One may pick up" , "One making purchases" , "Customer" .)

I've seen this clue in the Evening Standard.
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