Casual observer turned out to be a Serb spy (6-2)

I believe the answer is:

passer-by

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'casual observer turned' is the definition.
I can't tell whether this definition defines the answer.

'out to be a serb spy' is the wordplay.
'out to' indicates an anagram.
'a'+'serb'+'spy'='aserbspy'
'aserbspy' anagrammed gives 'PASSER-BY'.

Can you help me to learn more?

(Other definitions for passer-by that I've seen before include "One happening past" , "Witness" , "Person who happens to be going past" , "One walking near" , "Someone coming near" .)

I've seen this clue in The Globe and Mail.

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