Crossword Genius

Puzzle as to how one can get cross after misspelling 'purple' (7)

Ross

I believe the answer is:

perplex

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'puzzle' is the definition.
(synonyms)

'as to how one can get cross after misspelling purple' is the wordplay.
I cannot really understand how this works, but
'cross' could be 'x' (an X is a cross) and 'x' is present in the answer.
'as to' could be 'per' (I've seen this in another clue) and 'per' is found in the answer.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for perplex that I've seen before include "Nonplus" , "Dumbfound" , "Bewilder" , "Baffle, flummox" , "Bamboozle" .)

I've seen this clue in the Evening Standard.
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