Crossword Genius

Arbitrarily develop this fake reaction to someone's appearance (5,2,3,5)

Ross

I believe the answer is:

speak of the devil

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'reaction to someone's appearance' is the definition.
I know nothing about this answer so I can't tell whether this works.

'arbitrarily develop this fake' is the wordplay.
'arbitrarily' indicates anagramming the letters (I've seen this in other clues).
'develop'+'this'+'fake'='developthisfake'
'developthisfake' with letters rearranged gives 'SPEAK OF THE DEVIL'.

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I've seen this clue in The Independent.
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